Written by Ma. Disa Ricafort

The circulatory system! It is the system responsible for the circulation of blood around the body. This system comprises the heart and an incredible number of vessels that carry blood to every extremity of your body.

There is a lot to discuss here. Let’s begin by having a close peek at the heart.

We often use the heart as the “symbol of love,” “seed of our soul,” or the “core of our being.” Sorry to burst your bubble, but no. That is not the heart’s business. It does not make you love. It doesn’t break apart if you get dumped.

Yet, your heart is still remarkable. It keeps you alive. It is the “engine of life.” Even if it is about the size of your fist, it can pump blood to your body via a network of vessels to bring nutrients and hormones to your cells.

Take a small part of the heart as shown in the picture and zoom it in. You’ll have a closer look at the wall of the heart. The heart wall itself has several layers. It has three of them. The epicardium is the outermost layer. The others are the myocardium and endocardium. Each of these plays a definite but different role in the body.

Let’s go through all these in more detail, starting with the endocardium.

Endocardium

As its name suggests, the endocardium is the innermost layer of the wall of the heart. This layer serves as a barrier that all the blood cells are bumping up against. It houses the heart’s conduction system, which makes your heart pump.

The endocardium has three sublayers that define its function. These are as follows:

• Endothelium

This sublayer regulates material exchange between circulation and the heart muscles. Specialized endothelial cells make up this structure. It is very similar in many ways to the inner lining of the blood vessels,

• Fibroelastic tissue layer with smooth muscle cells
• Subendocardial layer

It is the endocardium’s outermost sublayer that connects to the heart muscle. In addition to nerves and arteries, it contains fibrous collagen cells that give structure and stability. It also has Purkinje fibers that send electrical impulses to the myocardium.

Given its vital function, endocardium disorders can have serious health consequences.

The most noteworthy of them is endocarditis. It is the inflammation of the inner chambers and valves of the heart. It has two types. One of them is the infective type. It is more common and caused by bacteria or others. The other one is non-infective; you can get this type by mechanical stress or chemical agents. Whatever the cause is, this can be fatal.

Myocardium

The myocardium is the heart’s principal functional element.

It consists of muscles or muscle fibers that allow the heart to contract. But you will find a certain amount of connective tissue as well. It is the thickest of the three layers, and its thickness varies across the heart. The myocardium in the ventricles is thicker than in the atria.

This is because the myocardium in the atria has only two layers. It consists of a superficial layer of muscle fibers arranged circularly and a deep layer. The latter consists of longitudinal muscle fibers, forming the inner muscle. This inner muscle is the pectinate muscle.

Meanwhile, the ventricles have three. We have the superficial muscle layer and the longitudinal muscle layer. Between those, there is a middle layer of circular muscle fiber.

You will notice that the left ventricles have the thickest myocardium. Why is that so? The reason is that it needs more strength to give a powerful contraction, for it pumps blood to the entire body.

Cardiomyocytes make up the myocardium. These muscle cells look different but still contract like others. They are shorter with fewer nuclei than skeletal muscle cells. It also has striations, like skeletal muscle tissue.

Epicardium

It also goes by its other name, visceral pericardium.

Since its primary function is protection, it consists of the following:

• Mesothelial cells. These are the same cells as in the parietal pericardium.
• A layer of connective tissue. Elastic fibers and fatty tissue comprise this layer. It provides direct contact with the epicardium to the myocardium.

To add to its function, the epicardium aids in producing the pericardial fluid. This fluid decreases friction between the layers, helping the heart pump more smoothly.

You can also find the coronary vessels and nerves that supply the heart in this layer.

What is the histology of heart valves?

Now, let’s go into more detail about heart valves.

Heart valves. They are the one-way mini gates between your heart chambers. They open and close to enable blood to circulate. They make sure that it goes from the atria into the ventricles and not vice versa.

There are two types: atrioventricular and semilunar valves.

Atrioventricular (AV) Valves

A fibrous structure separates the atria and the ventricles into two functional units. The said structure is made from endocardium and connective tissue. Embedded within these fibrous structures are the atrioventricular valves.

Each valve consists of an aperture enclosed by a ring and two or three leaflets that extend to shut the opening.

Histologically, three distinct layers of connective tissue comprise the said leaflets. These are as follows:

• Atrialis layer – made of elastin; contained more elastic fibers than the ventricularis
• Spongiosa layer – contains sparsely cells embedded in ground substance, made of glycosaminoglycans
• Fibrosa layer – enriched with large bundles of fibrous Type I collagen

We have strands mostly made of collagen and elastin, called chordae tendineae. It is also colloquially known as the heartstrings. It anchors the leaflets to papillary muscles that keep the flaps tight. These strands prevent them from everting back into the atria when our heart is under strain from pumping.

The tricuspid and mitral valves are the right and left atrioventricular valves, respectively.

Tricuspid valve

As the name implies, it consists of three irregularly shaped cusps or flaps. The cusps comprise endocardium folds attached to the heartstrings.

Each flap has many heartstrings connected with it. It consists of around 80% collagen fibers, and the rest comprises elastic fibers and endothelium. It binds each flap to a papillary muscle extending from the inferior ventricular surface.

The leaflets are named based on the margin or the papillary muscle they are attached to. Thus, there are septal, anterior, and posterior muscles and similarly named cusps of the valves.

Mitral or bicuspid valve

It is named as such for it has two flaps. Like in the tricuspid, the bicuspid valve is anchored by the heartstrings. But it is more robust and thicker. The left ventricle requires more power to pump blood under high pressure.

Semilunar valves

They have a half-moon shape, thus the term “semilunar.” These valves are the “doorways” that prevent backflow into the heart. As the ventricles relax, the blood will flow back from the arteries and press against its cusps, forcing them to close.

There are two of these valves. On the right or the pulmonary side, there is the pulmonary valve. Meanwhile, on the aorta side, there is the aortic semilunar valve.

They share the same construction as AV valves. Yet, unlike AV valves, semilunar valves lack heartstrings and papillary muscles. Instead, these valves comprise cusps made up of endocardium supported with connective tissue.

The semilunar valves also comprise fibrosa and spongiosa layers. However, it has a ventricularis layer rather than an atrialis layer.

• Ventricularis layer – composed of radially oriented elastin with a trace of collagen
•  Spongiosa layer
• Fibrosa layer

The semilunar valves consist of the pulmonary and aortic valves. These valves separate the ventricles from the pulmonary artery and aorta.

Pulmonary valve

This valve has three leaflets, separating the right ventricle from the pulmonary artery. This functions to prevent the blood from flowing back to the right ventricle.

It is positioned in an oblique plane, pointing in a posterior and superior manner toward the left- hand side. At the origin of the pulmonary artery, the pulmonary valve’s cusps are connected to the half-moon arches of the cardiac skeleton.

Aortic valve

This valve has three leaflets: the left, the right, and the non-coronary cusp (named after its well- defined sinuses). These cusps prevent the backflow from the aorta to the left ventricle.

The aortic valve lacks a continuous collagenous ring. Instead, there are three fibrous, triangular arches. These structures act as attachment sites for the cusps.

Moreover, the aortic and mitral valves interact with one another. As left ventricle contraction happens, the mitral valve shuts; meanwhile, the aortic valve opens. This allows the blood to flow via the aorta and then out to the body parts.

What is the histology of arteries?

An artery is a large thick-walled muscular vessel distributing blood to an area. It has all three layers (also known as tunics) of blood vessels (except capillaries). These tunics surround the open space, or lumen, that holds the blood.

The three tunics or layers are described further below.

Tunica Intima

Your circulatory underwear. It comprises an endothelium and is continuous with the lining of the heart. The endothelium consists of simple squamous epithelium tissue. A delicate elastic and collagenous layer of variable thickness supports it.

Tunica Media

The middle layer surrounds the tunica intima. It consists of a layer composed of variable smooth muscle cells and elastin ratios. Nerve fibers govern the smooth muscles, allowing them to constrict or dilate. That makes the tunica media an essential part since it plays a crucial role in blood flow and pressure.

Tunica Externa

The coat of your vessels. This layer is made of loosely woven collagen fibers to protect and reinforce the whole blood vessel.

Since they are closer to the heart and receive blood flowing at a much higher pressure, arteries have thicker walls than veins. In addition, arteries have narrower lumina than veins. These narrow lumina help keep blood pressure constant as it moves through the system.

As a result, arteries appear to have a rounded appearance in cross-section cuts.

What is the histology of a vein?

There is a series of veins and venules to return blood to the heart. The veins still have the same fundamental layers: tunica externa, tunica media, and tunica intima.

But in comparison to arteries, veins have proportionately less elastic and muscular components. This is because it does not need pressure on the blood to flow. Moreover, veins have much wider lumens and thinner walls than corresponding arteries.

The layering in the venular wall is not as precise as it is in arteries. The tunica intima is relatively thin. Only the larger veins include significant subendothelial connective tissue. Internal and exterior elastic laminae are either missing or thin.

The tunica media is thinner than the tunica externa. Under the microscope, it seems that the two layers tend to blend.

Furthermore, the morphological appearance of the vein wall is also affected by its location. The walls of veins in the lower extremities usually are thicker than those in the upper parts of your body.

Veins, unlike arteries, have valves to prevent backflow against the force of gravity. Arteries do not need any valves because of the intense pressure from the heart that makes blood flow in one direction.

What is the function of arteriole?

Arterioles. These are the smaller versions of your arteries. They share a similar structure with the arteries, containing all three tunics. But as they get smaller and thinner, they end up being mostly a single layer of smooth muscle surrounding endothelial cells.

They regulate the flow from those high-pressured arteries into the tiny capillaries. They dramatically slow the flow of blood. Hence, a maximum pressure drop is mainly seen in the arterioles. That is why arterioles also go by their other name, the resistance vessels.

Amazing, right? How does it work?

Arterioles can actively respond to physical stimuli. They alter blood flow as it goes into our capillary beds via vasodilation and vasoconstriction of their smooth muscle.

When there is excessive intravascular pressure, they constrict and maintain a smaller diameter. Meanwhile, they dilate when the blood flow increases and becomes broader and open.

What is the function of vasa Vasorum?

Vasa vasorum are the specialized vessels discovered in your vessel walls. The role of vasa vasorum in your body is to give you the nutrients and oxygen needed for your blood vessel walls.

Also, it eliminates waste products generated by the cells in the walls of your vessels. When diffusion from your luminal surface fails to meet the nutritional demands, the vasa vasorum externa steps in.

Vasa vasorum from your tunica externa develops into your tunica media of big arteries and veins. It actively controls the blood flow to your vessel wall.

Vasa vasorum proliferates into your intima-media of atherosclerotic arteries. This part is where vasa vasorum offers sustenance into your thickened artery. Anyhow, your neovascular channels have weak walls. This may lead to intraplaque bleeding, plaque rupture, and mural thrombosis.

What is the difference between arteriole and venule?

Arterioles and venules are the smaller versions of arteries and veins. These vessels transport your blood to and from your capillary beds. Your arterioles connect your arteries and capillaries.

Meanwhile, your venules connect your capillaries and veins.

They also differ in their sizes.

Arterioles are 0.01-0.3mm in diameter. They consist of a single smooth muscle layer overlapping endothelial cells when they are near your capillaries. On the other hand, venules range from 8 to 100 mm in diameter. They have a thin tunica externa and a tunica media consisting of two or three layers of smooth muscle cells.

Furthermore, arterioles are the ones that carry your blood that is rich in oxygen. They deliver blood from the left side of your heart into your smallest vessels, your capillaries. In contrast, the venules are the ones that carry your blood with low oxygen from your capillaries back to the right side of your heart.

Another difference is that the lumina of your venule are substantially more prominent. It also has thinner walls than your arterioles.

What is the difference between arterioles and capillaries?

Arterioles are the ones that connect your arteries and capillaries. Meanwhile, capillaries are the ones that touch your arteries to your veins. In addition, arterioles distribute blood rich in oxygen into your capillaries. In contrast, capillaries return waste-rich blood into your venules connected to your veins and the vena cava.

Your arterioles and capillaries also differ in the number of their tunics. The arterioles have three layers: the tunica externa, tunica media, and tunica intima, while the walls in your capillaries only have one. Capillaries only have tunica intima, which is made entirely of endothelial cells.

Arterioles control the flow into your capillaries by vasoconstriction and vasodilation.

On the other hand, capillaries use diffusion to allow an exchange of substances. They enable oxygen and nutrients to diffuse out of your blood into your tissues. At the same time, they are allowing carbon dioxide and wastes to pass from your tissues into your blood.

References

Betts, G. J. (2013, March 6). Heart Anatomy – Anatomy and Physiology. Pressbooks. https://opentextbc.ca/anatomyandphysiologyopenstax/chapter/heart-anatomy/

Blood Vessels. (n.d.). Www.cliffsnotes.com. Retrieved May 27, 2022, from https://www.cliffsnotes.com/study- guides/anatomy-and-physiology/the-cardiovascular-system/blood-vessels

Blood Vessels, Arteries, Capillaries, Veins, Vena Cava, Central Veins | LHSC. (2018). Lhsc.on.ca. https://www.lhsc.on.ca/critical-care-trauma-centre/blood-vessels-arteries-capillaries-veins-vena-cava-central- veins

Brelje, T., & Sorenson, R. (n.d.). Arterioles and Venules | Cardiovascular System. Histologyguide.com. Retrieved May 27, 2022, from https://histologyguide.com//slideview/MH-024-025-mesentery/09-slide-1.html

Heart Valves and Heart Sounds. (2021, July 27). ThoughtCo. https://www.thoughtco.com/anatomy-of-the-heart- valves-373203

human cardiovascular system – Origin and development. (n.d.). Encyclopedia Britannica. https://www.britannica.com/science/human-cardiovascular-system/Origin-and-development#ref84798

Leaflet valve. (2013). ScienceDirect. https://www.sciencedirect.com/topics/engineering/leaflet-valve

Libretexts. (2020, August 14). 17.1C: Layers of the Heart Walls. Medicine LibreTexts. https://med.libretexts.org/Bookshelves/Anatomy_and_Physiology/Book%3A_Anatomy_and_Physiology_(Bo undless)/17%3A_Cardiovascular_System%3A_The_Heart/17.1%3A_The_Heart/17.1C%3A_Layers_of_the_Heart_ Walls

Mbbs, L. B. C. (2022, April 5). Heart valves. Kenhub. https://www.kenhub.com/en/library/anatomy/heart-valves

Mulligan-Kehoe, M. J., & Simons, M. (2014). Vasa Vasorum in Normal and Diseased Arteries. Circulation, 129(24), 2557–2566. https://doi.org/10.1161/circulationaha.113.007189

Nursing Times. (2018, March 26). Vascular system 1: anatomy and physiology | Nursing Times. Nursing Times. https://www.nursingtimes.net/clinical-archive/cardiovascular-clinical-archive/vascular-system-1-anatomy- and-physiology-26-03-2018/

The Heart Wall Is Made up of 3 Layers That Have Their Own Functions. (2019, May 11). ThoughtCo. https://www.thoughtco.com/the-heart-wall- 4022792#:%7E:text=The%20heart%20wall%20is%20composed,epicardium%2C%20myocardium%2C%20and%20en docardium

Ritman, E. L., & Lerman, A. (2007). The Dynamic Vasa Vasorum. Cardiovascular Research, 75(4), 649–658. https://doi.org/10.1016/j.cardiores.2007.06.020 Williams, J. K., & Heistad, D. D. (1996). Structure and function of vasa vasorum. Trends in Cardiovascular Medicine, 6(2), 53–57. https://doi.org/10.1016/1050-1738(96)00008-4

Written by Sheariah R. Torrillo

What are the major tissues of the nervous system? These cells are your neurons and glial cells. Your neurons are responsible for communicating through electrical signals. While your glial cells, also supporting cells, maintains the environment around your neurons.

The nervous system has a vast array of functions. Being complex in nature, it can control your body’s internal environment to promote homeostasis and regulation. Even being complex, it only has two major types of cells in its nerve tissue for significant tissues.

Your neurons, also known as your nerve cells, are cells under polarization. These cells propagate the functions of your nervous system. They do this by conducting certain nerve impulses. These cells are the basis of the nervous tissue and are responsible for many things. It includes the communication and computation of the nervous system. They can release chemical signals that target cells and are electrically active.

Neurons are also accountable for such electrical signals. It allows the communication of information about sensations. Not only that, but it will enable the production of movement. It responds to certain stimuli and induces thought processes within your brain.

A significant factor in your neurons’ function is their shape and structure. Together with their three-dimensional shape, these cells can make diverse connections. Such connections are within the nervous system, which makes them possible. These specialized cells are in high amounts and are amitotic too. It can no longer be replaced if one neuron is in destruction since neurons do not undergo mitosis.

The structure of a typical neuron contains three essential parts. It includes your cell body or soma, a single axon, and one or two dendrites. The cell body has a nucleus and at least one nucleolus. It has typical cytoplasmic organelles but lacks centrioles. Dendrites also referred to as fibers, are cytoplasmic extensions and axons. It projects from your cell body and is sometimes branching and short.

This structure increases their surface area to receive significant signals from other neurons. When it comes to its number within a neuron, it varies. Known as afferent processes, it transmits impulses towards the neuron cell body. Only one axon is present when projecting from each cell body. Known as the efferent process, it elongates in shape. It also carries impulses away from your cell body.

Meanwhile, your glial cells play a supporting role in your nervous tissue. It is also known as glial or neuroglial cells. It also has a structural function within your central nervous system. It regulates immune response, nerve firing rates, and brain plasticity. Glial cells exist in both your peripheral nervous system and central nervous system. Scientists have shown that glial cells are your brain’s other half in functionality.

Its size has two types, macroglia, and microglia. Your macroglia, also known as more giant glial cells, can help neurons develop. It also migrates, insulates, and protects them. Your microglia is also known as your small type of glia. It has phagocytic properties when digesting foreign particles.

Types of glial cells are also present. It includes astrocytes, Oligodendrocytes, Schwann cells, Ependymal cells, Radial glia, and microglia. They all have significant functions for your central nervous system and glial cells.

How are neurons classified?

Neurons classify themselves in the functionality of signals along with their structure. It includes how specific signals travel about your central nervous system. It results in your neurons divided into three different types. These types are your sensory neurons, interneurons, and motor neurons.

Sensory neurons, also known as your afferent neurons, are unipolar. It transmits information from the sensory receptors of your skin. It also scoped your internal organs towards your central nervous system for processing. Interneurons are extensively involved in the integration of signals. This type of neuron is present between your sensory and motor pathways. The majority of this type of cell is in confinement within your central nervous system.

Motor neurons, also known as your efferent neurons, are multipolar. It transmits information away from your central nervous system towards a particular type of effector.

Neurons are classified in their structure. Its basis is on the number of processes extending out from your cell body. Neurons are then classified into three major groups. It includes neurons that are multipolar, bipolar, and unipolar.

Multipolar neurons are the significant type of neurons in your central nervous system. It is also the efferent division of your peripheral nervous system. This type of neuron has three or more processes that extend out from your cell body. IN humans, they comprise more than 99% of their neurons.

Bipolar neurons are rare and are present in your olfactory system and the retina of your eye. This type of neuron only has two processes that extend in opposite directions from your cell body. One process is your dendrite, and the other process is your axon.

Unipolar neurons are present in the afferent division of your peripheral nervous system. This type of neuron has a single and short process. It extends from your cell body and branches into two or more processes. These processes then extend in opposite directions. The peripheral process is also known as the process that extends peripherally. It associates with your sensory reception. The process that extends toward your central nervous system is significant.

What is the difference between an axon and a dendrite?

Axons carry nerve impulses away from your cell body. At the same time, dendrites carry nerve impulses from synapses to your cell body. Both your axon and dendrites are two components of your nerve cells. It functions to maintain nerve impulses to your spinal cord, brain, and body. This is so to coordinate certain functions.

For its structure, axons are long and thread-like. In contrast, dendrites are short branched extensions of your nerve cells. A nerve cell has only one axon but many dendrites. Axons arise from an axon hillock. It is also knowns as a conical projection, and dendrites arise direct from your nerve cell. Axons are very long and branched at their ends, while dendrites are very short and branched all along. Axons can either be myelinated or non-myelinated, while dendrites are non-myelinated.

Axons are from the efferent component of your nerve impulse. In comparison, dendrites are from the afferent part of your nerve impulse. For axons, the tips of the terminal branches are extensive in the form to form synaptic knobs. At the same time, no synaptic knobs are present at the ends of the components of your dendrites. Synaptic knobs for your axon contain vesicles. These vesicles have neurotransmitters, while dendrites do not have vesicles that contain neurotransmitters.

What is a Schwann cell?

A Schwann cell, also known as your neurilemma cell, is a glial cell. It is under your peripheral nervous system. It aids in separating and insulating your nerve cells. This type of cell also can produce the myelin sheath around neuronal axons.

This type of cell has its name after Theodor Schwann. He is a German physiologist who discovered Schwann cells during the 19th century. Schwann cells are equal to oligodendrocytes. It is a type of neuroglia in your central nervous system.

Schwann cells stimulate themselves to increase some constituents of the axonal surface. In instances where motor neurons are severed, causing nerve terminals degenerate. Schwann cells act to occupy the original neuronal space. When it comes to degeneration and regeneration, Schwann cells that remain after the generated nerve determines the route.

When it comes to the course and process of degeneration, which follows regeneration, fibers regenerate in a way that they go back to their original target sites.

What is the difference between myelin sheath and Schwann cell?

Schwann cells wrap around the axon of your neuron to form your myelin sheath. Your myelin sheath serves as an electrically insulating layer. Schwann cells and the myelin sheath are two types of structures in the axon of your neuron.

At the same time, Schwann cells produce myelin, while myelin can increase the speed signal of transmission. Schwann cells are also glial cells. It swaddles around the nerve fiber of your peripheral nervous system. It forms the myelin sheaths of your peripheral axons. These are cells that wrap around the neuron’s axon and secrete myelin.

On the other hand, your myelin sheath is an insulating covering surrounding the axon. It also has various spiral layers of the myelin. It is discontinuous at the nodes of Ranvier and increases the speed at which a nerve impulse can travel along an axon. It consists of myelinating Schwann cells. It then serves as an electrical insulator. This insulator speeds up the signal transmission through your neurons.

How does myelination differ in the CNS and PNS?

Myelin is present in both your central and peripheral nervous systems. But, only the central nervous system is affected by the myelin sheath. In the central nervous system, myelin is fabricated by oligodendrocytes that are considered to be special cells. In the peripheral nervous system, myelin is generated by Schwann cells.

Two types of myelin, as mentioned, are chemically different, but both perform the same function—both function towards promoting the efficient transmission of a nerve impulse along the axon.

Different glial cell types make myelin differently, depending on their location. Schwann cells make myelin in your peripheral nervous system. At the same time, oligodendrocytes are in your central nervous system. One Schwann cell forms a single myelin sheath in the peripheral nervous system. Your

oligodendrocytes send cell processes towards myelinate multiple segments on many axons in the central nervous system.

There are several morphological and molecular variations between nerve fibers in the central nervous system and the peripheral nervous system for nerve fibers. However, the basic myelin sheath arrangement and the electrophysiological characteristics are the same.

What is a node of Ranvier?

The nodes of Ranvier are a periodic gap in the insulating sheath on the axon of specific neurons. Myelin-sheath gaps are particular axonal segments that lack myelin. It functions in facilitating the rapid conduction of nerve impulses. Louis-Antoine Ranvier first discovered this type of myelin cover. He is a French histologist and pathologist who first discovered it in 1878.

When it comes to the composition of the myelin sheath, it has concentric layers of lipids. It includes cholesterol and a possible amount of phospholipids and cerebrosides. Thin layers of protein also separate these. This arrangement gives way to a high-resistance and low-capacitance electrical insulator.

However, your nodes of Ranvier interrupt the insulation at intervals. This discontinuity enables certain impulses to jump from node to node. This process is saltatory conduction.

What forms the nodes of Ranvier?

The nodes of Ranvier are characterized by specialized and short regions in the axonal membrane that is not insulated by myelin. The nodes contain high concentrations of voltage-gated sodium ion channels. It is responsible for raising the membrane voltage during creating an action potential that is all-or-nothing.

The nodes of Ranvier, which also include adjacent regions, have high concentrations of the structural proteins ankyrins. It is responsible for the anchorage of proteins towards the axonal cytoskeleton. In terms of pathological distribution of essential nodal proteins, it can lead to dysfunction in the propagation of nerve impulses. It also includes ion channels and cell adhesion molecules.

The node of Ranvier is highly organized in terms of its structure and molecular composition. It stems from the cell borders of its neighboring Schwann cells. It also forms the insulating myelin sheath around axons of a larger caliber.

The node of Ranvier is a site of excitation. It is full of Nav channels, also known as voltage-gated Na channels. These channels concentrate on the node. But it has little presence under the myelin sheath. From a physiologic point of view, the node of Ranvier is the component of the fiber present. It handles the action potential’s generation and propagation.

The nodes of Ranvier are approximately one μm wide. It also exposes the neuron membrane to external environments. These gaps are full of ion channels that mediate the exchange of specific ions.

It includes chloride and sodium, which requires forming an action potential. An action potential is the reversal of the electrical polarization of the neuron membrane. It initiates and is a part of a wave of excitation that travels along the axon.

When it comes to the action potential, it propagates by one node of Ranvier. It also reestablishes at the next node along the axon. It enables the action potential to travel at a pace along the fiber.

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Written by Mary Abbygale Cabahug

Blood vessels are the conduits that transport blood throughout your body. They form a closed loop that starts and stops at your heart, similar to a circuit. Your heart vessels and blood vessels make up your circulatory system. Your body has around 60,000 miles of blood veins.

Endothelial cells, smooth muscle cells, and extracellular matrix make up blood vessels. It includes your arteries and veins (including collagen and elastin). There are three concentric layers (or tunica): intima, medium, and adventitia.

 The intima (or tunica intima)

The innermost layer of the vein is what we all know as the tunica intima. Flat epithelial cells make up this stratum. These cells allow fluid to flow freely and have valves to keep the flow in one direction. This continuous layer of epithelial cells within the vascular lumen contains cells and fluid.

The tunica intima’s thin outer layer contains a small amount of areolar connective tissue. It is mainly made up of elastic fibers to give the vessel more flexibility and some collagenous fibers to give it more strength.

Any trauma to the tunica intima might cause an inflammatory reaction. This can result in platelet aggregation and thrombosis.

The media (or tunica media)

The tunica medium, or middle layer, is the thickest part of the wall. The sympathetic nervous system innervates it, which is primarily smooth muscle. The sympathetic nervous system induces the venospasms in reaction to changes in temperature or irritation within the vein.

It’s also made mainly of thin, cylindrical, smooth muscle cells and elastic tissue. It makes up the majority of the wall of most arteries. Smooth muscle cells are grouped in circular layers around the vessel, and the coating thickness varies with vessel size.

Smooth muscle contraction results from alpha-receptor stimulation. Beta receptor stimulation, on the other hand, causes vessel dilatation. As a result, sympathetic modulation of blood pressure is possible. Furthermore, the smooth muscle layer secretes the extracellular matrix.

The adventitia (or tunica adventitia)

The adventitia or tunica externa is the outermost layer of the blood vessel wall. It is connective tissue, as well as vasa and Nervi vasorum, that makes up this layer. It is essential for vascular health.

It is the most powerful of the three layers. It’s made up of collagenous and elastic fibers. (Collagen is a protein present in connective tissue.) The tunica adventitia works as a limiting barrier, preventing the vessel from overexpanding.

What are the three main differences between arteries and veins?

Arteries and veins are two types of blood arteries in the circulatory system. They are primarily responsible for circulating blood throughout the body. Despite these similarities, the two blood vessels behave very differently.

The primary distinctions between your arteries and veins are that:

The arteries are in charge of transporting oxygenated blood away from the heart to various organs. On the other hand, Veins transport deoxygenated blood from multiple organs of the body to the heart for oxygenation.

You can locate your arteries deeper within your body and have thick elastic muscle walls. Veins have thin, non-elastic, less muscular walls, and most of them are closer to your skin’s surface.

The direction of blood flow for arteries is downward from the heart to the body tissues. The veins carry blood from the body tissues to the heart upward.

How are arteries and veins similar?

Even while your arteries and veins serve different functions in the body, they are comparable in several ways:

Type of Blood Vessel

Arteries and veins are components of the body’s circulatory system. These veins keep the blood flowing and deliver oxygenated blood to the body. Furthermore, they return deoxygenated blood for purification. They provide nutrition, hormones, and nourishment to the body in addition to

blood circulation. The circulatory system also aids in illness prevention and body temperature regulation.

Transportation of Blood

The natural mechanism by which the body transports oxygen to the organs and returns carbon dioxide is blood transportation. It takes two heartbeats for your veins and arteries to complete one circulation cycle. You will stay healthy and active if your blood flow is good. Nonetheless, your body is prone to developing blood vessel diseases that impair blood circulation.

Arteries and Veins have different layers.

Arteries and veins are made up of different layers of cells that keep the system together. These veins are malleable and transport nutrients to the body. Three layers make up the framework of these vessels.

The innermost layer is the initial layer, the tunica intima. It comprises several capillaries that connect the veins to the connective tissues.

Tunica media is the middle layer. It is a thick layer that keeps blood pressure stable. The tunica externa is the outermost layer of veins and arteries. It contains connective fibers and protects the vessels.

However, veins have valves in their innermost layer, the tunica intima, to direct blood flow.

One directional blood transportation

Both of these vessels are moving in the same direction. These vessels’ function is to keep the blood flowing ahead. Vessels only carry blood from the heart to the organs. On the other hand, Veins transport blood to the heart for purification.

How do you identify blood vessels in histology?

You can easily see blood vessels with hematoxylin and eosin stains on light microscopy. You can also identify blood vessels through the thickness of blood artery walls. Arteries have three layers of strong walls (tunica). Veins have thin walls but a larger lumen (lumen size may vary depending on the specific artery or vein).

The morphological differences between arteries, capillaries, and veins are due to their different functions.

Because they transmit blood under tremendous pressure, arteries have thick walls and limited lumens.

Because capillaries exchange resources between blood and tissue, their walls are only a single cell thick. Because veins convey blood at low pressure, they feature thin walls with broad lumens and valves.

Why are veins thinner than arteries?

Veins, like arteries, have three layers. Despite all these layers, you can find less smooth muscle and connective tissue here. Vein walls are thinner than artery walls because the blood in veins has lower pressure than blood in arteries.

Because blood does not exert pressure on vein walls, they are thin. Your veins carry the rest of your body’s blood back to your heart. The walls of your vein walls are way thinner than artery walls because the pressure of blood returning to the heart is relatively low. Your blood vessels need thick walls because blood flow puts great pressure on artery walls.

Are veins bigger than arteries?

The blood in your arteries travels more swiftly. Your arteries are thicker and stretchier to resist blood pressure. Your veins are smaller and less flexible. This configuration allows veins to transport more blood for longer than arteries.

On the other hand, Veins have greater diameters, carry higher blood volume, and have thinner walls concerning their lumen. Arteries are smaller than veins, have thicker walls involving their lumen, and transport blood at a higher pressure. Arteries and veins frequently travel in pairs, sharing connective tissue routes.

What are the two types of veins?

Your body consists of two types of veins. The pulmonary and systemic vein.

Pulmonary veins. Pulmonary veins are major blood channels that transport oxygenated blood from the lungs to the rest of the body. There are four pulmonary veins in total, two from each lung, left and right, that drain into the heart’s left atrium.

Each lung has two pulmonary veins that arise from the hilus. These pulmonary veins get blood from 3-4 bronchial veins before draining into the left atrium. The pulmonary veins that attach to the pericardium run alongside the pulmonary arteries.

Unlike most veins, pulmonary veins carry oxygenated blood from tissues to the heart. The pulmonary veins drain into the left atrium and return oxygenated blood from the lungs to the heart. After the left atrium pumps blood through the mitral valve into the left ventricle, the blood oxygenates and circulates to the body’s organs and tissues through the aorta.

Systemic veins. The systemic circuit transports deoxygenated blood back to the heart, which is oxygenated via the pulmonary circuit. The systemic veins rule.

Systemic veins can be further divided into two categories:

• Deep veins: These veins are frequently in muscular tissue and have a corresponding artery nearby. A one-way valve in these veins may prevent blood from flowing backward.
• Superficial veins: These veins are close to the skin’s surface and do not have an adjacent artery with the same name. A one-way valve may also be present.
• Connecting veins: Blood can travel from the superficial veins to the deep veins through these little veins.

Do veins have valves?

Veins, unlike arteries, have valves that ensure blood only travels in one way. Arteries don’t need valves since the heart’s pressure is so high that blood can only flow in one way. Valves also assist blood is returning to the heart against gravity.

The valves in most veins open and close. Blood flow is controlled by valves, which keep blood flowing in one direction. Your veins contain about 75% of your blood.

To help prevent blood backflow, small crescent-shaped flaps of tissue called valves are sprinkled throughout your veins. These valves are angled towards the heart and project from the vein wall’s innermost layer to the vein’s center in the direction of venous blood flow.

Because they lack the muscles to open and close doors, they work passively. When blood rushes through a valve, it opens it, and when the blood flow slows, it closes it.

References:

Aryal, S. (2021, November 19). Difference between Arteries and Veins. Microbiology Info.Com.

Retrieved March 22, 2022, from https://microbiologyinfo.com/difference-between- arteries-and-veins/

Brennan, D. (2021, May 21). The Difference Between Arteries and Veins. WebMD. Retrieved March 23, 2022, from https://www.webmd.com/heart/difference-between-arteries-and- veins#:%7E:text=Your%20arteries%20are%20thicker%20and,a%20longer%20time%20t han%20arteries.

B.S., A. F., MD PhD. (2021, October 28). Pulmonary arteries and veins. Kenhub. Retrieved March 23, 2022, from https://www.kenhub.com/en/library/anatomy/pulmonary-arteries- and-veins

BYJU’S Admin. (2021, March 22). General Data Protection Regulation(GDPR) Guidelines BYJU’S. BYJUS. Retrieved March 22, 2022, from https://byjus.com/biology/arteries-and- veins-difference/

Chideckel, N. (2021, July 1). Veins & Arteries Similarities And Differences. Top Varicose Vein Treatment NYC. Retrieved March 23, 2022, from https://www.varicoseveintreatmentnyc.com/veins-arteries-similarities-and- differences/#:%7E:text=Arteries%20and%20veins%20form%20a,deoxygenated%20bloo d%20back%20for%20purification.

Cornell, B. (2016). Vessel Comparison | BioNinja. ninja. Retrieved March 23, 2022, from https://ib.bioninja.com.au/standard-level/topic-6-human-physiology/62-the-blood- system/vessel-comparison.html

Seladi-Schulman, J., PhD. (2018, April 14). Venous System Overview. Healthline. Retrieved March 23, 2022, from https://www.healthline.com/health/venous-system#types-of-veins

Written by Ysandra Prille A. Tabilon

Fibrous connective tissue (FCT) is the most diverse type of connective tissue in your body. It is also referred to as fibroconnective tissue or connective tissue proper. It is usually located in the muscles, tendons, bones, ligaments, and skin, among other places.

As its name implies, it contains a lot of prominent fibers. To be precise, it consists of Collagen, Reticular, and Elastic fibers. Based on the relative quantity of these fibers, they can be loose or dense connective tissue.

The majority of the collagen fibers are in a parallel pattern. This type of fiber is well-known for strengthening and stabilizing your body. It is present in your muscles, bones, and other parts of your body that offer structural support.

Other than fibers, it also consists of a few cells and little ground substance. Its cells include fibroblasts, macrophages, leukocytes, plasma cells, mast cells, and adipocytes.

In general, it serves to support and absorb shock in your bones, tissues, and organs. It also provides strength to the inner layer of your skin. Thus, allowing it to withstand the stresses associated with joint movements.

This high-strength and stretchy tissue also help to absorb movement’s shock. As a result, it assists in protecting vital organs and tissues. There would be damage to your organs if they were not supported by fibrous connective tissue.

What are the connective tissue fiber types?

Connective tissue consists of three distinct types of fibers. What you need to know are the following:

Collagen fibers. They are fibrous proteins composed of type 1 collagen. They are the strongest and most abundant of all connective tissue fibers. They are often referred to as white fibers. It is due to the sparkling white appearance of new collagen fibers in tendons.

This fiber is the primary component of your tendons, cartilage, ligaments, and dermis. It is then secreted into the extracellular space. Here they supply your tissues with varying degrees of strength and rigidity. Additionally, they are flexible and have a high tensile strength that resists stretching.

Elastic fibers. They contain the elastin and fibrillin proteins. These long, thin fibers form a branching network within the extracellular matrix. They are usually referred to as yellow fibers due to the yellowish color of new elastic fibers.

These fibers exhibit a unique property known as elastic recoil. Hence, they resemble coiled metal springs. This is because they can recoil to their original shape upon stretching.

They are predominant in the walls of large blood vessels and elastic cartilages. They are also present in yellow ligaments, the lungs, the urinary bladder, and the skin.

Reticular fibers. These fibers also referred to as argyrophilic fibers, consist of collagen type III. They are short, fine collagenous fibers covered with glycoproteins. They branch to form a delicate mesh-like network in organs such as the spleen, kidneys, and lymph nodes.

They are prevalent in areas where connective tissue connects to other tissues. This includes the basement membrane of epithelial tissues, capillary endothelial, and muscle fibers. Also, they provide structural support for your liver and lymphoid organs, among others.

These fibers, in general, form a functional and structural unit that supports tissues. Its disorganized structure also allows for molecular mobility within the extracellular fluid.

What does collagen do to your body?

Collagen is your body’s most abundant protein. Connective tissue cells secrete it, and it’s present in the extracellular matrix. This protein then assists in the formation of fibroblasts. This is a fibrous network of cells on which new cells can grow.

It helps to build and support your body components by forming fibers. This applies to your bones, cartilage, muscles, skin, hair, eyes, and others. Some also serve as protective coverings for delicate organs like the kidneys. It is not only beneficial but also essential for your health and wellness.

This protein comes in various forms, each with its own set of properties and functions. The four most common collagen types are as follows:

Type I. The most common and plentiful amount of this protein. It is present in connective tissue. This includes your skin, ligaments, teeth, bones, and tendons.

Type II. It is present in cartilages that form tough, flexible tissues in body parts. This includes your joints, intervertebral discs, ears, and nose.

Type III. The main component of reticular fibers. It is present in your skin, intestines, and blood vessels. It helps with blood clots and healing wounds.

Type IV. It is a component of your kidneys, inner ears, and eye lens.

As people age, there will be a considerable decline in collagen production. This leads to wrinkles, weaker cartilage in joints, and other changes. This problem can be alarming to others who are conscious of their appearance and health.

If you are one of those individuals, have no fear. Collagen possesses a variety of health benefits. It helps in the replacement and restoration of your dead skin cells. It is famous in the cosmetic business for its ability to rejuvenate the skin and make you appear younger. These supplements have risen in popularity as a result.

But, other than that, this protein has other health benefits for your body. This includes the following:

• Keep bones strong and healthy (prevent bone loss and reduce Osteoarthritis pain)
• Promote Healthy joints (relieve joint aches and pain)
• Promotes skin elasticity and hydration (skin revitalization)
• Boost muscle mass (increase body and muscle mass)
• Promotes wound healing and new tissue growth

Other potential health benefits are also mentioned. But, there hasn’t been much research into them. It includes the following:

• Thicker hair
• Healthier and stronger nails
• Weight loss (may speed up metabolism and promote weight loss)
• Promote Gut health
• Promote Brain health (may reduce anxiety and improve mood)
• Promote heart health (may reduce the risk of heart conditions)

Is it good to take collagen?

Celebrities and social media keep on marketing collagen supplements. Your friends may even be gushing about how taking it has improved the appearance of their skin and hair. But, the primary question is this: Is it good for you?

At present, studies and debates on collagen’s safety and effectiveness are still ongoing. But, so far, it appears to have a wide range of potential health benefits. These supplements are unlikely to harm you, but they aren’t always necessary either.

Whether we take collagen supplements or not, our bodies produce it naturally due to the foods we eat. This includes ingesting foods rich in vitamin C, zinc, copper, glycine, and proline.

Supplements may provide you with increased levels of some amino acids, but not all. Healthy eating still works better than relying on a supplement.

It’s your choice if you want to buy these supplements. They are available in tablets, capsules, and powder forms. It is already hydrolyzed for it to absorb in your body easier. Depending on your needs, you can choose from a selection of supplement types.

According to healthcare professionals, taking supplements is good and generally safe. Most people don’t experience adverse side effects. But, others may still experience mild sideeffects such as diarrhea, rashes, and others. Please consult your physician before taking a new supplement or increasing its usage.

Should you take collagen every day?

Since your body produces collagen, supplements may not be necessary. You may choose to do so to gain benefits or treat conditions like collagen deficiency. The amount of a supplement to take depends on its form and purpose.

Depending on the form of the supplement:

• Hydrolyzed collagen: 2.5-15 grams of it each day may be effective for skin, bone, and hair health.
• Undenatured collagen: 10-40 milligrams per day can improve joint health.
• Gelatin: 1-2 tablespoons of powder or 1-2 pieces of a pill/gummy is recommended daily

These servings can differ in collagen content depending on the supplement. Hence, there is a need to look over the recommended daily dosage on the instruction. Also, it is vital to follow any instructions provided.

Depending on the benefit:

• Skin and Hair: 2.5 to 10 grams per day can be beneficial for skin and hair health
• Muscle: 15 to 20 grams per day can aid in muscle mass, muscle strength, body composition
• Joint: 2.5 to 5 grams a day may help joint support
• Bone: 5 grams per day improves bone density

Currently, there is no official specification for the correct usage of this protein. The optimal dose and frequency of administration are unknown at the moment. But research indicates that using supplements on a daily basis is acceptable.

Some individuals take between 2 and 15 grams of collagen daily for at least 12 weeks. This protein is generally considered to be a safe and nontoxic daily supplement. The majority of people will experience no adverse effects.

Yet, ingesting an excessive amount may result in unwanted side effects. This includes an unappealing aftertaste and an increased sense of fullness. Thus, it is imperative to consult your physician on proper collagen intake.

Does collagen make you gain weight?

A lot of people believe collagen contributes to weight gain. It may be because it is the body’s most abundant protein. But, as a matter of fact, collagen does not cause weight gain. Rather than that, it is a crucial tool for helping you maintain a healthy weight. This is how

It reduces body fat (Metabolism support).

This protein is a powerful tool for building lean muscle mass. Building lean muscle mass helps your body burn calories, supporting your metabolism. This is why this protein is popular among fitness and health influencers. It helps keep metabolism healthy to burn fats.

It helps you feel full.

Taking collagen helps you feel full and suppresses appetite hormones such as ghrelin. This allows you to consume less food, thus eliminating unnecessary calories. In exchange, you will increase your body’s fat-burning capacity.

It enhances workout readiness.

Supplement of this protein contributes to a faster recovery time following an exercise.

Additionally, it alleviates aching muscles and joint pain and helps prevent workout-related injuries. It also helps in the healing process if you have an open wound. This allows you to return to the gym the following day and continue to lose pounds.

Along with proper diet and exercise, collagen can aid you in your weight loss journey.

Besides that, it can also help you deal with the adverse side effects of weight reduction. A daily dose of one to two powder/gelatin scoops can already do wonders for your body.

Can collagen make your hair grow?

Collagen does not directly stimulate or speed hair growth. But instead, it has an effect on the health of your hair follicle, which is essential for optimal growth. It performs this by lining the inner sack of the hair follicle and the dermis from which a hair grows. It then strengthens and elasticizes its structure.

Hair thinning or hair loss is a natural part of aging. This occurs due to natural collagen depletion or damage to the follicle. Taking collagen provides your body with the building blocks needed to produce hair.

It also promotes the health of the skin on the scalp, resulting in healthier, thicker hair. Other than that, it also has many other health benefits for your hair, including the following:

• Provides amino acids used to build hair
• Help prevent damage to hair follicles
• Help to prevent hair thinning caused by aging
• Help slow graying of hair
• Hydrates the hair and scalp
• Provides antioxidant support
• Improves overall appearance of the hair
• Add volume and makes the hair shiny

As with any supplement, individual results may vary. It’s worth taking supplements for several months to see if it improves hair health. Take caution not to overdose with supplements since this can cause adverse effects.

Does collagen have side effects?

In general, collagen supplements are well tolerated and appear to be safe for most people.

Yet, there are a few reports of adverse reactions. These supplements may cause mild side effects, which may include the following:

• Diarrhea. This occurs when the GIT is having a hard time breaking down supplements.
• Constipation. This is due to too much protein and insufficient fluid and fiber in the gut.
• Bloating. This occurs due to many gases in the stomach
• Feeling of fullness. Collagen impacts appetite and intake after meals leading to a decrease in appetite
• Allergic Reactions. Some supplements contain dairy or shellfish products that cause allergic reactions to others.
• An unpleasant taste. Some may experience a bad taste in their mouth following consumption.
• Kidney stone. Hydroxyproline in collagen peptides may pose a risk and trigger kidney stone formation.
• Hypercalcemia. Supplements may raise calcium to an unhealthy level.
• Affects mood. This may cause mood alterations leading to anxiety, depression, nervousness, and irritability
• Skin breakouts or Rashes. Possible inflammation due to the formula’s preservatives, additives, and heavy metals.

Some individuals have reported other adverse side effects, including:

• bone pain
• fatigue
• heart arrhythmia

Before taking supplements, discuss your health history with your physician. If you’re pregnant or breastfeeding, you may want to avoid taking supplements for a while. This is because there isn’t enough research to draw conclusions about their safety.

References:

Baron, A. & Enzor, L. (2021, October 19) What is Fibrous Connective Tissue? from https://study.com/learn/lesson/fibrous-connective-tissue.html

Carter, A. (2019, June 1) What to know about collagen supplements. MedicalNewsToday.

Characteristics of Connective Tissue. (2020, August 14) from libretexts

Duggan, C. (2022, February 22) Collagen Peptides for Hair: Benefits and How to Use. Byrdie from byrdie/collagen peptides for hair

Engler, A. (2022, March 28) How Much Collagen Should You Take Per Day? Experts Explain Dosage, Results & Benefits from a Mind Body Green article.

FACE Med Store (n.d.) What are the Side Effects of Collagen Peptides? from FACE Med Store blog.

Fibrous Connective Tissue: Function & Types. (2013, May 22) from Study.com

Jacob, D. (2021, May 6) How Much Collagen Should You Take a Day? from MedicineNet.

Kane (2022, April 11) Does Taking Collagen Make You Gain Weight? from the The Mode Mag.

McIntosh, J. (2017, June 16) What is collagen, and why do people use it? from MedicalNewsToday.

One Ocean Beauty (2021, April 1) Does Collagen Cause Weight Gain? from One Ocean Beauty blog.

Snyder, C. (2020, September 18) How Much Collagen Should You Take per Day? from Healthline.

Streit, L. (2019, January 23) 5 Evidence-Based Ways Collagen May Improve Your Hair. from Healthline.

The hair fuel (2018, December 5) Collagen Pills don’t improve hair growth and here is why. The Hair Fuel.

Tonye A. Ogele, Fibrous Connective Tissue. OpenStax CNX. Nov 20, 2013 from cnx

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Underferth, D. (2021, July 1) Should I take a collagen supplement? MD Anderson. from mdanderson

Written by Jose Emmanuel Cisneros

The three types of cartilage in the body include elastic, hyaline, and fibrocartilage. These three tissues differ in their strength, body location, and extracellular matrix (ECM).

Cartilage is a connective tissue denser than the blood but less dense than the bone. It has chondrocytes that produce extracellular matrix (ECM). It contributes to the growth of cells and tissues. These materials lie in spaces called lacunae housing eight chondrocytes each.

This connective flesh in the body is flexible and avascular. Thus, this tissue can support, cushion organs, and aid body locomotion. 

At a microscopic level, each body tissue has its particular characteristic. Its difference in appearance and (sometimes) function aid histologists in determining their importance.

1. Elastic Cartilage      

It also goes by its other name, yellow cartilage. The elastic fiber and collagen fiber network contribute to its color. These fibers usually give off yellow color. And this is all thanks to the principal protein making up the fiber called elastin.

This one resembles the structure of hyaline cartilage. But the only difference is the number of elastic fibers: the elastic has more of this. This characteristic enables the tissue to have great flexibility that withstands continuous bending.

2. Hyaline Cartilage

It encompasses most of the cartilages of the body. They consist of a translucent protoplasm studded with one or two round nuclei. Sometimes, you can see interlacing filaments on the granulated matrix. 

The hyaline contains collagen fibers the most, attributing to the glass-like appearance. It has no nerve or blood vessel network. This function makes it a perfect cushion for covering different organs.

3. Fibrocartilage or Fibrous Cartilage   

This tissue contains the most collagen fiber among the three, with Type I and Type II. It has a mixture of fibrous connective tissue and cartilaginous tissue.

The fibrous tissue gives it flexibility and toughness. Meanwhile, the cartilaginous tissue gives it elasticity. The fibrocartilage belongs to the densest cartilage among the three. It is also the only cartilage having type I collagen besides type II collagen.

How does cartilage form?

A cartilaginous tissue contains chondrocytes, fibers, and extracellular matrix (ECM). These materials help in cartilage formation by attracting water. It would then give its characteristic shape and property. 

Chondrocytes

They produce collagen, proteoglycans, glycoproteins, and hyaluronan. These macromolecules constitute the shape and structure of the tissue.

Chondrocytes vary among the different types of cartilaginous tissues. It depends on how much matrix the cell produces. The chondrocytes clump up together and form cell nests within the cavities of the tissue. 

Extracellular Matrix (ECM)

This constituent differentiates connective tissues from others. They originate from the secretions of the chondrocytes. They also have two elements: the fibers and the ground substance.

The definition of the fibers will appear in the following section. The ground substance contains water, adhesion proteins, and polysaccharide molecule. 

The cell adhesions serve as a glue in function. They enable the attachment of connective tissue cells (chondrocytes) to the matrix fibers.

Meanwhile, the polysaccharides help trap water as they become more complex structures. The more the polysaccharides become abundant, the more they make the matrix fuller. The number of polysaccharides determines the density of the tissue. Their consistency ranges from fluid to gel-like.

Fibers

The fibers found in cartilages include collagens (white), elastic (yellow), and reticular fibers. Collagen contributes to its high tensile strength. Meanwhile, elastic fiber aids in stretching and recoiling motion. Reticular fibers create a cushion for soft tissues.

The monomers of these fibers originate from the chondrocytes of the tissue. They link together like other proteins and carbohydrates to form these fibers.

All these substances contribute to the shape and density of the connective tissue.  The following shows the order of cartilages (from less dense to densest):

Hyaline Cartilage ➡️ Elastic Cartilage ➡️ Fibrocartilage

Where are they located?

In general, cartilages live most in the tips of body organs. This way, they would have more range in their cushioning and connecting abilities. Here are some lists of body parts where we can find the three types of this connective tissue: 

·         Hyaline Cartilages

This cartilaginous flesh has the most amount in the body. One reason that contributes to this attribute points to the procreation of the body. It comprises the early embryonic skeleton of a baby. Examples include:

• the bone ends in free moving-joints
• rib ends
• nose
• larynx
• bronchi
• trachea

·         Elastic Cartilages

The more pliable of the three, it resides in parts where it undergoes tension. It also helps in bouncing off vibrations traveling in the specific body canal. Example body parts are:

• epiglottis (near the larynx region)
• larynx
• pinnae (external ear location)
• the auditory tube of the middle ear (Eustachian tube)

·         Fibrocartilages (Fibrous Cartilages)

The densest cartilaginous tissue of the bunch. This asset helps protect and cushion organs sensitive to impact (like the bones). Some organs with fibrocartilage are:

• intervertebral disks (spinal column)
• ligaments
• tendons
• pubis symphysis (pelvis location)
• the joint between the manubrium and sternum (rib location)
• temporal mandibular joint (lower jaw location)
• menisci (knee location)

What is the strongest cartilage?

According to doctors, the strongest cartilage found is fibrocartilage. The reason for this leads to its contents. It has a thick layer of alternating structures. You can relate to this when you fix a bed with different layers of quilts and blankets.

These alternating components constitute layers of hyaline cartilage matrix and dense collagen fibers. They orient a direction that compensates for the pressure on the connective tissue.

It means that its layer arrangement counters the stresses placed on the body organ. This attribute also helps to pad the organs that experience the hardest friction. — vertebrae, knee, long bones.   

Does cartilage affect height?  

Yes, cartilages can indeed influence the growth of height among people. This event happens when a structure in the bone named growth plate aids in bone elongation. It then points to why children will not stay small and short in their lifetime.

A growth plate is a layer of cartilaginous tissue found in most long bones of the body. Most of the cartilages found in the growth plate belong to the category of hyaline. These hyaline connective tissues provide the growth of bones and soon the height.

The way this tissue affects the height of bones occurs in two ways:

1. An embryonic bone will use hyaline connective tissues as its model for growth. They develop through chondrogenesis. The soft hyaline will continue to serve as the temporary bone for the developing body.

• Then as the fetus matures, the hyaline will turn to bones with the help of osteoblasts in the bone matrix. The cartilages digest away and leave a medullary cavity within the bone. By birth, the connective hyaline disappears— except for two structures. 

The hyaline undergoes more synthesis and osteoblast action. As this continues, more bones will form and pile up after another. The ending result of this leads to notable growth in bone length. When the bone length increases, the height follows.  

Which growth plates determine height?

The epiphyseal plate pertains to the growth plate that determines the height. This hyaline structure can still produce a chondrocyte-producing matrix in the bone. Thus, it can help the long bone achieve longitudinal growth.

The growth plate exists between the epiphyseal and metaphyseal bones. A closer look reveals three zones on the growth plate. These zones contribute to the differentiation of cartilaginous cells helpful for bone formation. They are:

A.      Resting Zone

This area has many small chondrocytes that act like stem cells. It also plays a role in protein synthesis and germinal structure maintenance. A slow replication rate happens in this area.

B.      Proliferative Zone

This area contains flat chondrocytes lining the long axis of the bone. Production of collagen (Types II and XI) also happens here. The replication rate here speeds up in preparation for bone formation.

C.      Hypertrophic Zone

Here, the differentiation of chondrocytes finishes. And you can notice this due to its increased thickness. It also houses a calcified matrix and the formation of bone and vessels. Maturation of bone happens in this zone.

Can HGH make you taller at 18?

No, HGH will not work anymore at the age of 18. Doing so will only cause malfunctions in the Human Growth Hormone levels. Everyone should remember this implication, as it will save their lives.

To put it short, growth in the body and bones stops during adolescence. This time marks the ages of 16-18 years. The growth plate of these ages has now completed its work and converted it into a bone structure.

No chondrogenesis and ossification will happen anymore if this occurs. It is because the plate has already calcified as a spongy bone. All that remains in that area hosts the remnants of the epiphyseal plate. This structure alludes to a bony structure known as the epiphyseal line.

As for the HGH, it cannot play any role anymore in height determination. The bone has now finished its growth. High doses of HGH will only thicken the bone instead of making them increase in length.

HGH now will only work on producing muscle and bone for body organs and cell maintenance. Enlargement of organs and insulin regulation are some actions that HGH can still do now.

Adults that keep supplying HGH in their bodies may sport slight height increases. But these may not even compare to the growth most adolescent children experience. Some may even call this unnatural.

Thus, using HGH to increase height at 18 will not work in most cases.

Is it possible to increase height after 21?

It depends on the development of your bones. If you see that the growth plates on your longs bones still exist, your height can still grow. But there will be no growth with an already-closed growth plate.

For those people with a still-growing growth plate, they still have hope. They can increase their height by also increasing their HGH. This scenario can happen but only in a few cases like delayed growth spurts or a body disorder.

But most cases would have no chance to increase their height when they reach ages 18-21. The growth plates at this point close off.

Still, do not let this discourage you. Eat healthy and nutritious food and strengthen your bones through exercise. You can also wear clothing that would make you appear tall. Positivity in life is one key to looking tall.

What causes cartilage loss?

The cartilages lose their strength when overworked. Friction, mechanical stress, and destructive substances render the connective tissues weak. And in time, the fibers and matrix that hold the cartilaginous mass break down.

There exist many reasons why our cartilages break down. From our body chemistry to the lifestyle we adopt, in particular. All can influence the entire anatomy and physiology of these structures. Here are some causes of cartilage-related loss:

1. Osteoarthritis

This disorder categorizes as a degenerative disease. So, as you grow older, the cartilaginous fibers in your joints also get old and degrade. They have now worn down because your body has used them ever since.

2. Trauma

Sudden traumatic events like injuries or accidents cause weakening in the connective flesh. Atrophy will soon follow in that area, leading to a faster breakdown. These areas should expect easier damage because of their avascularity.

3. Joint Instability

Cartilage-induced damage can cause joints to be loose and movable. It leads to extra motion in the tissue. If they move too much, it can injure that area. Arthritis will soon follow and start degradation of cartilages.

4. Aging and Genes

As mentioned before, aging will weaken the area of concern and reduce the fibers/matrix. Passing faulty genes could also account for the loss of the connective flesh. It happens to a considerable number of people. Even so, both factors induce loss in cartilaginous mass.

5. Lack of Exercise

A sedentary lifestyle will make the joints lose strength. Some cases could also lead to atrophy of joints. Thus, exercises should appear to keep the joints away from under usage.

6. Biomechanics and Poor Alignment

An asymmetrical cartilage loss accounts for joint degradation. You may have a body-alignment issue that is causing certain joints to wear down more than others.

7. Obesity

Obesity harms knee joints and increases the load on knee joints by 2-4 times. An increased risk of knee replacement can also happen.

8. Metabolic Disorders

The chemical matrix structure of joints can destabilize by metabolic syndrome. Furthermore, adipose (fat) tissue can overproduce leptin. It is a hormone that regulates a sense of fullness. But it also has an impact on bone and cartilage. Greater leptin leads to more arthritis over time.

9. Poor Nutrition

Poor dietary habits and nutrition deficiency damages your cartilage and musculoskeletal system. What you eat has the power to turn on or off the genes that protect your cartilages.

10. Medication

The drugs injected into arthritic joints for pain treatment also hazards the cartilage. An example of this is epinephrine, which has an acidic (low pH) nature. Apoptosis is the death of cartilage cells caused by anesthetics and steroid medicines.

These only represent a decent number of reasons for cartilage loss. Due to its wide range of disorders, you should be careful of your every action. Your joints and cartilaginous flesh are also important, like your cardiac muscle (heart). (2,219 words)

REFERENCES:

Akkiraju, H., & Nohe, A. (2015). Role of Chondrocytes in Cartilage Formation, Progression of Osteoarthritis and Cartilage Regeneration. J. Dev Biol., 3(4): 177–192. Published online 2015 Dec 18. doi: 10.3390/jdb3040177.

Britannica, T. E. (2020, Spetember 24). cartilage. Retrieved from Encyclopedia Britannica: https://www.britannica.com/science/cartilage

Chang, J. (2018). Growth considerations in pediatric upper extremity trauma and reconstruction. Plastic Surgery: Hand and Upper Extremity, (6).

Higuera, V. (2019, January 18). What do growth hormone injections do? Retrieved from Medical News Today: https://www.medicalnewstoday.com/articles/312905#why_use_human_growth_hormone

Jacob, D. (2021, November 24). Is It Possible to Increase Height After 21? 6 Height Factors. Retrieved from MedicineNet: https://www.medicinenet.com/is_it_possible_to_increase_height_after_21/article.htm

Jee, Y., & Baron, J. (2016). The Biology of Stature. J. Pediatr., 173: 32–38. Published online 2016 Mar 26. doi: 10.1016/j.jpeds.2016.02.068.

Leiber, J. (2019, February 18). THE MOST COMMON CAUSES OF CARTILAGE LOSS. Retrieved from Regenexx: https://newregenortho.com/the-most-common-causes-of-cartilage-loss/

Lui, J. C., Nilsson, O., & Baron, J. (2014). Recent research on the growth plate: Recent insights into the regulation of the growth plate. J. Mol. Endocrinol., 53:T1–T9.

Maes, C., & Kronenberg, H. M. (2012). Postnatal bone growth: growth plate biology, bone formation, and remodeling. . Pediatric bone, 2nd ed. San Diego: Elsevier Co.; 2012. pp. 55–82.

Marieb, E. N., & Keller, S. M. (2018). Cells and Tissues. In E. N. Marieb, & S. M. Keller, Essentials of Human Anatomy & Physiology (12th Edition) (p. 120). New York City: Pearson.

Marieb, E. N., & Keller, S. M. (2018). The Skeletal System. In E. N. Marieb, & S. M. Keller, Essentials of Human Anatomy & Physiology (12th Edition) (pp. 167-168). New York City: Elsevier.

Metro MD. (2015, September 3). HGH for Height in Adults after 18 Years Old. Retrieved from Metro MD.com: https://www.metro-md.com/hgh-for-height-in-adults-after-18-years-old/#:~:text=Therefore%2C%20simply%20utilizing%20HGH%20to,that%20are%20not%20yet%20closed.

Nall, R. (2018, January 16). What Is the Purpose of Cartilage? Retrieved from Healthline: https://www.healthline.com/health/cartilage#types

Nillson, O., Marino, R., De Luca, F., Phillip, M., & Baron, J. (2005). Endocrine regulation of the growth plate. Horm. Res., 64:157–165.

Parvizi, J. (2010). Cartilage. In J. Parvizi, & G. K. Kim, High Yield Orthopaeducs. Philadelphia: Saunders.

Saltzman, B. M. (2021). Cartilage 101 – What Is Cartilage and What Does It Do? Retrieved from OrthoCarolina: https://www.orthocarolina.com/media/cartilage-101–what-is-cartilage-and-what-does-it-do#:~:text=What%20is%20cartilage%20made%20of,and%20other%20non%2Dcollagenous%20proteins.

Structure, Type, and Location of Cartilage. (2020, May 26). Retrieved from Medicine LibreTexts: https://med.libretexts.org/Bookshelves/Anatomy_and_Physiology/Book%3A_Anatomy_and_Physiology_(Boundless)/6%3A_Skeletal_System/6.2%3A_Cartilage/6.2A%3A_Structure_Type_and_Location_of_Cartilage

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University of Leeds. (n.d.). Cartilage: The three types of cartilage. Retrieved from The Histology Guide: https://www.histology.leeds.ac.uk/bone/cartilage_types.php

Watkins, J. (2014). Fibrocartilage. In A. Brahme, Comprehensive Biomedical Physics. Stockholm: Elsevier.

Written by Elisha Kristin Pasco

The cardiac muscles are also known as the myocardium. They make up the muscular middle layer of your heart and enable it to circulate the blood in your body. The myocardium is a muscle that you can only locate in your heart. Surrounding it is a thin outer layer called visceral pericardium or your epicardium. Your myocardium then covers the inner layer called the endocardium.

The myocardium is responsible for the involuntary contraction and relaxation of your heart. It can make your heartbeat because of cardiomyocytes. Cardiomyocytes are cardiac muscles that compose your myocardium. The primary function of these heart cells is to contract, thus enabling your heart to pump your blood.

What are the main characteristics of cardiac muscle?

The cardiomyocytes that make up your myocardium have a rectangular shape. They are branching cells with only one nucleus found at the center. It also contains mitochondria. The mitochondria provide your heart with the energy needed for contraction.

The cardiomyocytes need a sufficient amount of adenosine triphosphate. It is why mitochondria are present within the cells.

When cardiomyocytes organize themselves into repetitive units, it forms a sarcomere. The sarcomere is the repeated overlapping of the muscle filaments. The thick and thin myofilaments arrangement gives your myocardium its signature striated appearance.

Sarcomeres, however, are not exclusive to only your myocardium. They are also present in your skeletal muscles. And, like in the myocardium, they are also responsible for their striated appearance.

A sarcolemma surrounds your cardiomyocyte. It is a plasma membrane that regulates what comes in and out of the cells. The sarcolemma contains invaginations called T-tubules. The tubules hold the numerous proteins necessary for cardiomyocyte function. The proteins found within the cavity are as follows:

• L-type calcium channels

• sodium-calcium exchanger

• calcium ATPases

• beta-adrenergic receptors

The t-tubules are invaginations that excite the contraction of your cells. They ensure that your body’s pump can circulate the blood in your body.

Intercalated discs link each cardiomyocyte through three different cell junctions. The discs are a distinct feature of your heart tissues and contain the following:

• fascia adherens
• desmosomes,
• gap junctions.

Fascia adherens serve as anchoring junctions. They enable actin filaments to attach to the thin filaments of your sarcomeres to your cells.

Desmosomes are adhesion sites that keep your muscles cells together during a contraction.

Gap junctions are responsible for direct contact between the cells of your heart. It enables electric communication, enabling your heart to beat.

The myocardium also holds pacemaker cells called sinoatrial (SA) nodes. The SA nodes are responsible for regulating your heart rate.

What is the structure of cardiac muscle?

The myocardium is composed of individual muscle cells called cardiomyocytes. Cardiomyocytes can make your heart contract due to the myofibrils that make up the cell. Your myofibrils are the specialized cytoskeletal structure that enables your heart to beat.

As a cytoskeletal structure, your myofibrils help the cardiomyocytes maintain their shape. They are composed of myofilaments. They are rod-like tubules that overlap and organize themselves in repeating units called sarcomeres.

Sarcomeres are the contractile units of your muscle cells. Two types of myofilaments overlap to form your sarcomeres. Thick and thin myofilaments containing unique proteins make your sarcomeres.

The thick myofilaments contain the protein myosin, whereas the thin ones hold actin.

Actin is the major cytoskeletal protein of your cardiomyocytes. It is the protein responsible for the cells’ movement. On the other hand, myosin is a protein that converts adenosine triphosphate (ATP) to energy. It provides your muscles the fuel to move.

Is cardiac muscle voluntary or involuntary?

The cardiac muscle is involuntary. AF Huxley and R Niedergerke and HE Huxley and J Hanson’s first described the sliding filament theory. They discussed their theory in two research papers published in 1954. The papers explain how the muscles in your heart can beat on their own.

The research describes the molecular basis behind the muscle contraction of your heart. In the paper, it notes that the sarcomeres have two different zones. The “A band” is an area that maintains its length during the contraction. On the other hand, the “I band” is the zone that changes its span when the sarcomere contracts.

The A band contains thick myofilaments composed of myosin. The constant length of this zone suggests that though the myosin participates in the beating of your heart, it is central and does not move. The I band is thin actin filaments that shorten whenever the heart contracts. Observations between the myosin and actin during the contraction of your heart enabled the development of the sliding filament theory.

The sliding filament theory describes actin sliding past the myosin. The movement creates tension within your muscles. This action would cause the sarcomere to shorten because actin is bound to the z bands. Z bands are structures at the lateral end of your sarcomeres. It transmits the tension from one sarcomere to the next.

Since the beating of your heart is involuntary, it would have to be regulated by some processes in your body. For your myocardium to contract, it uses calcium and ATP cofactors. ATP provides your muscles with the energy to move, whereas calcium is responsible for regulating muscle contraction.

Calcium, however, isn’t what controls the contraction of your heart. However, the proteins that manage it, troponin and tropomyosin, require it as a trigger.

The tropomyosin keeps the myosin from binding with the actin if the sarcomere is at rest. On the other hand, troponin is the protein that moves the tropomyosin from the myosin-binding sites of actin, enabling contraction. The action between troponin and tropomyosin is only possible when there is calcium. Without calcium, the myosin-binding areas will remain blocked by the tropomyosin, keeping actin and myosin from sliding against each other.

How do you identify cardiac muscle tissue?

The arrangement of the sarcomeres in your myocardium and striations can help you identify the cardiac muscle at first glance. However, skeletal muscles are also striated, so to further differentiate one from the other, look into its defining characteristics.

Aside from the striations, you will need to look into the general shape of the cells that make up your tissue. Cardiomyocytes are branching rectangular cells.

The next step would be to locate and count how many nuclei are present within the cells. Your myocardium is mononucleated. It only has one core situated in the center of the cell.

The striations in your heart are due to the arrangement of your thin and thick myofilaments in your sarcomeres. Your sarcomeres have different zones divided due to their composition and behavior during contraction.

The thickness of the A-bands gives them a darker color than the I-bands, with a relatively bright area in the middle. The Z disc presents itself as a dark line that connects the actin filaments of your muscles.

What is the shape of a cardiac muscle cell?

The myocardium is rectangular, but each muscle cell has a tubular structure. The shape is because of the repeating chains of myofibrils that form the sarcomeres, which have a rod-like profile.

What is the difference between skeletal, smooth and cardiac muscle?

Skeletal, smooth, and cardiac muscles differ in control, composition, shape, function, and location.

Skeletal Muscle

Skeletal muscles are attached to your bone and are in charge of your body’s skeletal movements and posture. Nerves from your somatic nervous system innervate the fibers enabling your central nervous system to control them.

As your central nervous system controls them, the muscles are under conscious and voluntary control. It is composed of repeating multinucleated cells that form sarcomeres. The sarcomeres are responsible for the striations in this muscle.

Aside from movement and posture, the skeletal muscles also play a hand in the following functions:

• Heat production

• Irritability – enables you to respond to stimuli from the external environment.

• Conductivity – can transmit impulses.

• Extensibility – gives you the ability to stretch without tearing yourself apart.

• Contractility – ability to shorten and create movement

Smooth Muscle

The smooth muscles are in the walls of hollow internal organs such as:

• blood vessels

• gastrointestinal tract

• urinary bladder

• uterus

You do not have voluntary control over your smooth muscles. Your autonomic nervous system controls them without your conscious input. Unlike your cardiac and skeletal muscles, the smooth muscles lack striations.

The lack of striations is because the thick and thin filaments do not form sarcomeres. Sarcomeres are responsible for the striated appearance of other tissues of your body.

Like the cardiac muscle, it can contract independently and with rhythm.

Cardiac Muscle

The cardiac muscles are muscles that are only present in your heart. The striations are due to the myofilament arranging themselves into sarcomeres as the skeletal muscle. The striations on the myocardium are shorter than those in your musculoskeletal system.

Unlike the musculoskeletal system, the cells of your cardiac muscle have one nucleus. The nucleus rests at the very center of the cell.

The autonomous nervous system controls them and enables them to contract involuntarily. The myocardium’s contractions have rhythm and are very strong.

References

Arackal, A., & Alsayouri, K. (2021, May 10). Histology, heart. NCBI Bookshelf. https://www.ncbi.nlm.nih.gov/books/NBK545143/.

Burgoyne, T., Morris, E. P., & Luther, P. K. (2015). Three-Dimensional structure of vertebrate muscle z-band: The small-square lattice z-band in rat cardiac muscle. Journal of Molecular Biology, 427(22). https://doi.org/10.1016/j.jmb.2015.08.018

Cooper, G. M. (2000a, January 1). Actin, myosin, and cell movement. NCBI Bookshelf. https://www.ncbi.nlm.nih.gov/books/NBK9961/

Guo, Y., & Pu, W. T. (2020). Cardiomyocyte maturation. Circulation Research, 126(8), 1086–1106. https://doi.org/10.1161/circresaha.119.315862

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Paxton, Steve, Knibbs, Adele, Peckham, & Michelle. (2003a, January 1). Muscle: The histology guide. University of Leeds. https://www.histology.leeds.ac.uk/tissue_types/muscle/Muscle_cell_junctions.php

Paxton, Steve, Knibbs, Adele, Peckham, & Michelle. (2003b, January 1). Muscle: The histology guide. University of Leeds. https://www.histology.leeds.ac.uk/tissue_types/muscle/muscle_cardiac.php

Saxton, A., Tariq, M. A., & Bordoni, B. (2021, August 11). Anatomy, thorax, cardiac muscle. NCBI Bookshelf. https://www.ncbi.nlm.nih.gov/books/NBK535355/

Sliding filament theory, sarcomere, muscle contraction, myosin. (n.d.-a). Learn Science at Scitable. Retrieved April 12, 2022, from https://www.nature.com/scitable/topicpage/the-sliding-filament-theory-of-muscle-contraction-14567666/

Szent-Györgyi, A. G. (1975). Calcium regulation of muscle contraction. Biophysical Journal, 15(7). https://doi.org/10.1016/S0006-3495(75)85849-8

Written by Naellah Yanca Marie P. Galve

The osteoblasts and osteoclasts are two types of cells found in the skeleton. The roles of osteoblasts and osteoclasts in skeletal maintenance are distinct. Osteoblasts are bone-builders, while osteoclasts are bone-eaters.

While both take part in repair, they differ in managing their function. Osteoblasts are responsible for growth and development. In contrast, osteoclasts play a role in the resorption and degradation of bony tissue.

Osteoblasts are uni-nucleated, cuboidal cells coming from the osteogenic cells in the periosteum. The periosteum is the tissue layer that protects the outermost surface. On the said surface, the osteoblasts appear as a densely-packed cell layer.

The rough ER, which manufactures and transports proteins, is abundant inside them. They also feature a prominent Golgi complex that packages the cell’s products. Their name “bone-forming cells” is due to their role in skeletal development.

They also aid in bone remodeling or healing. They secrete chemicals such as growth factors, osteocalcin, and collagen into the matrix. Alkaline and collagenase, enzymes necessary in bone- building, are also the product of osteoblasts.

These get trapped in the lacunae once the matrix surrounds the osteoblast. The lacunae are a collection of small, oblong spaces located between the lamellae. The entrapped cells develop into osteocytes, which are mature skeletal cells.

Osteocytes engage with the surface and receive nutrients through canaliculi. The canaliculi are long, slender transportation channels.

Osteoclasts arise from the monocytic cells or macrophages in the circulation. These are also present in the marrow cavity’s endosteum layer. The endosteum, a thin connective tissue, lines the inner medulla.

Osteoclasts are big and multinucleated. Their cytoplasm appears foamy and homogenous. They have microvilli, forming a brush-like structure extending to the active sites.

The “Howship lacunae” at the surface are where the osteoclasts are. Its cavity-like grooves are due to the enzymes secreted by osteoclasts. Examples are acid-phosphatases. These substances can dissolve the skeleton’s calcium, collagen, and phosphorus components.

As mentioned, the role of osteoclasts is to resorb bone to produce calcium. Bone resorption is the body’s response to low calcium levels in the blood. They secrete enzymes that break the bony complex down, releasing calcium into circulation. It is their primary function.

How do osteoblasts make bone?

Bones, of course, are essential body structures. Mobility is possible as they provide muscular attachment sites. For example, various bony structures protect organs, take the skull and ribs. Blood production, lipids, and mineral (e.g., calcium) storage are their functions.

We already know that osteoblasts are responsible for skeletal formation. This process, known as ossification or osteogenesis, means “production of new bones.” By producing a matrix that covers the surface of the older cells, they create new layers.

Ossification happens when mesenchymes and cartilages transform into bones. During the stages of development, most of the skeleton is cartilaginous. It allows the calcium to get deposited and grow, producing the body’s framework.

The two subtypes of ossification are intramembranous ossification and endochondral ossification.

1.      Intramembranous ossification

In the intramembranous ossification, the mesenchyme differentiates straight into the membrane. Examples of structures developed in such a way are the flat bones found in the skull.

The mesenchyme differentiates into osteoblasts and later begins to deposit osteoid. Osteoid is a term used to identify an unmineralized matrix rich in collagen. The bone-forming cells start depositing calcium phosphate into the osteoid tissue. This action aids osteoid maturation.

The osteoblasts transform to become osteocytes. The forming skeleton has no discernible pattern at first. The spicules become structured and merge into layers called lamellae. Around blood arteries, different lamellae grow, creating an osteon.

The osteon contains the Haversian canal system. Meanwhile, the other osteoblasts stay near the active site’s surface. They lay down lamellae that would later develop into compact bone. The intermediate bony structures between the surface plates remain porous.

It is worth noting that our skeletal system undergoes continuous remodeling until adulthood. As we live, bone cells die and get replaced. We can credit this to the coordinated functions of the osteoclasts and osteoblasts.

2.        Endochondral ossification

It entails the mesenchyme to produce cartilaginous models, then ossifies. Long bones, those in the extremities, vertebrae, and ribcage, come from cartilage.

Cell hypertrophy and calcium phosphate crystal formation are essential phases. Cell apoptosis and calcium matrix erosion are also inevitable. At the same time, a thin layer of tissue develops beneath the perichondrium. It causes it to transform into the periosteum, the outermost layer.

Ossification happens in two major “centers” called the primary and secondary ossification centers.

The diaphysis is where primary centers of ossification emerge. The process proceeds toward the epiphysis. The secondary centers appear in the epiphysis of long, bony structures in the first few years of life.

What are Osteons?

An osteon is a cylindrical structure that serves as the basic unit or building block of compact bone. The name osteon means “bone,” derived from its Greek origin. The primary functions of osteons include providing protection, structure, and strength.

Inside an osteon, we can find a mineral matrix and osteocyte units. The mineral hydroxyapatite makes up the skeletal matrix. It is high in calcium and phosphorus, as well as collagen.

Their cylindrical feature is due to the concentric layers, known as lamellae. Lamellae are round, concentric layers of tissue that surround a Haversian canal. They run parallel to its long axis. It is a beneficial feature as it helps in resisting stress or shock.

In the same layer, we can find the lacunae. The lacunae are chamber-like structures located between lamellae. It serves as the storage of mature osteocytes.

Where do osteoblasts reside?

The periosteum is the exterior layer, while the endosteum is the interior layer. Blood veins, nerves, and lymphatic tissue make up the periosteum. It is in charge of supplying nutrients to skeletal cells. Meanwhile, growth, healing, and remodeling occur in the endosteum.

Osteoblasts dwell in the periosteum’s innermost layer. Hence, they can add new layers inward to the endosteum. On the skeleton’s surface, they constitute a dense layer of cells. Their cellular processes extend throughout the active site, where it mineralizes the matrix.

The osteoblast formation occurs when osteogenic cells in the marrow’s periosteum & endosteum differentiate. It is also why osteoblasts are abundant in the connective tissues of these locations.

Do osteocytes have a Golgi apparatus?

Yes. Osteocytes have a Golgi apparatus along with other cellular organelles. An osteocyte is a type of cell found in mature skeletal tissue. They are unicellular with a stellate shape, their body sizing from 5-20 micrometers. They live in the cavities of osteons.

Osteocytes feature a single nucleus, a membrane, and a nucleolus or two. It also contains mitochondria, a smaller endoplasmic reticulum, and a Golgi apparatus.

We know that osteocytes are osteoblasts deposited in the matrix. They communicate through tiny canals called canaliculi. Nutrition and waste exchange happen in these canals.

Osteocytes are active in the turnover of the skeletal matrix. Also, they destroy bone using

osteocytic osteolysis. This process is faster and temporary compared to osteoclasts’ function.

Where is the Volkmann’s canal?

Perforating holes or channels, called Volkmann’s canals, are anatomic patterns in cortical bones. The Volkmann canals situate within osteons. They connect the Haversian canals to the periosteum and each other.

They have anastomosing vessels between Haversian capillaries and run to the Haversian canals. These tiny channels reach blood vessels from the periosteum to the inner surface. They are perforating canals that supply energy and nutrients to the osteons.

These canals serve to “perforate” lamellae and supply vessels for the core of osteons. Remnants of eroded osteons become irregular interstitial lamellae.

Do osteoblasts produce calcium?

No. Osteoblasts do not create calcium; instead, they deposit it. The osteoclasts, not the osteoblasts, are responsible for calcium production. Osteoclasts are responsible for breaking down the composite material in the skeleton. They do so using acid and collagenase proteins. Osteoclasts act on calcium in the bones, returning them to the bloodstream.

Providing calcium to the bloodstream is one of the skeletal system’s essential functions. Calcium is a vital nutrient for our bodies. Muscle contraction, blood clotting, nerve conduction, and other body functions need it. We use calcium to strengthen our teeth and, of course, the skeletal system.

Osteoclasts provide calcium through the degradation of bony material. The thyroid gland regulates osteoclast formation. When the need for more calcium arises in the blood, the osteoclast hormone activates. When calcium levels are normal, they become suppressed. They’re also crucial for healing skeletal fractures.

Osteoclasts are bone-degrading multinucleated monocyte-macrophage derivatives. They are vital to humans’ continuous removal and replacement of skeletal matter. Thus, skeletal material can manage extracellular calcium activity, which is crucial for survival.

They dissolve bony matter with acid secretions and proteinases, dissolving the collagen matrix. The calcium-rich environment permits the osteoclast to maintain homeostatic calcium levels. There is a removal of degraded products via membrane vesicles transcytosis.

Skeletal development is the function of unrelated stromal cell-derived osteoblasts. Thus, osteoclastic differentiation balances osteogenesis.

Interactions between osteoclast precursors and bone-forming cells govern osteoclast differentiation. The bony matter disintegrates into minute fragments, which the osteoclasts consume. Calcium and phosphorus get released into the bloodstream due to demineralization.

Where do dead bone cells go?

Human osteoclasts live for two weeks, osteoblasts for three months. Both experience apoptosis. Apoptosis, or programmed cell death, is how bone cells die. Apoptosis occurs in an estimated 50–70% of osteoblasts throughout rebuilding.

It is an essential component of embryogenesis and tissue morphogenesis. In adult skeletons, it contributes to physiological bone turnover, repair, and regeneration.

In a “zone of hypertrophy,” the developed chondrocytes in the lacunae undergo apoptosis. This zone is near the primary ossification center in the active site.

Osteoclasts die after eroding skeletal matter from the central axis in cortical bone. Phagocytes then remove these dead cells fast to make room for the new bony material. The majority of osteoblasts deposited at the remodeling site die as well.

The rest transforms into lining cells covering quiescent skeletal surfaces. They become encased osteocytes within the calcified matrix.

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Written by Ian Jay B. Francisco

Connective tissues occur throughout the body. They provide support, bind stuff together, and protect the body’s organs. Looking at a microscopic photo of connective tissues, you will see fibroblasts.

Connective tissues consist of cells, protein fibers, and a ground substance. Fibroblasts are the most common cell type that you can find in a connective tissue specimen. They produce and maintain the extracellular matrix of your connective tissues.

Fibroblasts secrete collagen proteins that support many tissues. They also help heal wounds. These cells are spindle-shaped, elongated, and have several processes that extend their bodies.

You can find them in the skin, tendons, and other tough tissues of the body. These are collagen-secreting cells. They can be grown in a lab for genotypic and phenotypic testing of the associated disease.

Like any other cell, organelles make up fibroblasts. They have an oval nucleus that is euchromatic. You would find an abundant endoplasmic reticulum and a well-developed Golgi apparatus. It is because they produce large amounts of protein.

The more minor, inactive form of this cell is the fibrocyte. The cells differ because the nuclei of fibrocytes are heterochromatic. Their cytoplasm and organelles are also lesser in comparison to fibroblasts.

Fibroblasts produce extracellular matrix (ECM) proteins, such as collagen and elastin. Aside from these proteins, the ECM also contains glycosaminoglycans, reticular fibers, and glycoproteins.

Contractile myofibroblasts, which are essential in wound healing, are unique fibroblasts. When tissue undergoes damage, fibrocytes become stimulated. They then undergo mitosis or multiplication by replication and division.

What is produced by fibroblasts?

Aside from tissue repair, fibroblasts have other functions. They are responsible for regulating and maintaining the body’s connective tissues. They achieve this by producing fibrous proteins and ground substances.

The ECM’s compositions are fibrous proteins and ground substances produced by fibroblasts. It provides an adjustable structural base for tissue growth and stores growth factors. The ECM also transmits signals within the tissue and acts as an adhesive substrate.

Fibrin, fibronectin, and collagen are the fibrous proteins produced by fibroblasts. Collagen provides the mechanical strength required for tissue formation. Meanwhile, fibrin and fibronectin give the basic framework for cell adhesion.

The ground substance is the collection of the ECM’s non-fibrous proteins. It is a clear gel that fills in cell gaps and helps tissues resist compression.

Glycosaminoglycans are long unbranched polysaccharide chains that you can find in-ground substances. An example would be proteoglycan. It plays a role in growth factor propagation and enzyme regulation.

Fibers and connector proteins produced by fibroblasts give tissues their structure. Reciprocal positive feedback regulation of these proteins can promote profibrotic myofibroblast differentiation. ECM structures can serve as lamina delineating borders separating distinct cell types

Fibroblasts secrete a diverse array of structural proteins with unique properties. Its rope-shaped triple-stranded helical tertiary protein structure reinforces its tensile strength. This structure also prevents overstretching.

Elastin proteins form cross-linked but unstructured elastic networks that stretch without breaking. Skin and lung tissues differ in their expression of collagen and elastin proteins. Moreover, fibrosis pathology includes an increase in the relative balance of collagen.

How do fibroblasts make collagen?

Like all other proteins, collagen’s components are amino acids. It supports extracellular connective tissue structure. Collagen synthesis occurs both inside and outside your fibroblasts.

Its rigidity and stretch resistance allow it to be a part of your skin, tendons, bones, and ligaments. Amino acids, which are the building blocks of proteins, also make up collagen. Collagen’s primary amino acid sequence is glycine-proline-X.

Collagen has three chains. The chains form a triple helix. Glycine allows the chain to create a closed configuration and withstand stress. Its synthesis usually occurs in fibroblasts.

Outlined below are the mechanisms involved in the synthesis of collagen.

Intracellular synthesis

mRNA transcription in the nucleus

• Transcription of genes for pro-a1 and pro-a2 chains.

Translation

• Translation requires mRNA to interact with ribosomes in the cytoplasm.
• In the endoplasmic reticulum (ER), it undergoes modification after translation.

Modifications after translation

• From there, the chain undergoes three alterations to create procollagen.
  1. Removal of N-terminal signal peptide
  2. Addition of hydroxyl groups by hydroxylase enzymes to lysine and proline residues
  3. Selective hydroxyl-lysine glycosylation with galactose and glucose b
• Three hydroxylated and glycosylated pro-a-chains form a triple helix through zipper folding. Three left-handed helices wrapped into a right-handed coil.
• The procollagen molecule now enters the Golgi apparatus for final alterations and assembly.

Extracellular synthesis

Cleavage of the propeptide

• Collagen peptidases are enzymes that cleave procollagen and turn it into tropocollagen.

Assembly of collagen fibril

• Tropocollagen molecules form covalent bonds between each other. The catalyst for this is a copper-dependent enzyme known as lysyl oxidase.

Collagen is the body’s most prevalent protein. So, it has various types. Collagen types I through V are the most frequent, each with distinct roles.

There are concerns about collagen’s biochemical production. Clinical symptoms of collagen synthesis errors exist. Scurvy, osteogenesis imperfecta, and Ehlers–Danlos syndrome are some notable disorders.

What do fibroblasts do in the heart?

Fibroblasts are important in heart development and remodeling. But, you might not know that they are crucial in cardiac anatomy and function. The cardiac fibroblast supports and maintains normal heart function.

With their regulatory function, cardiac fibroblasts coordinate communication between components of the heart. A cardiac fibroblast is a cell that makes connective tissue. Its ECM consists of collagens, proteoglycans, and glycoproteins, unlike bones and tendons.

These cells produce periostin, vimentin, fibronectin, and collagen types I, III, V, and VI. Although they are the prominent synthesizers, other cardiac cell types also produce these.

Studies show that cardiac fibroblasts respond to injury by generating ECM components. But their activities in uninjured hearts remain unknown. Endothelial cells are the most frequent non-cardiomyocyte cell type in the heart.

Although not the majority, fibroblasts still play a role in normal heart physiology. Their mechanisms include matrix degradation, conduction system insulation, and cardiomyocyte electrical coupling. Also involved are vascular maintenance and stress sensing.

Cardiac fibroblasts regulate the heart’s basal structure and take part in wound healing. After your heart receives damage, tissue-resident fibroblasts develop into disease-activated fibroblasts and myofibroblasts.

In the past, most fibroblast studies concerned markers and in vitro models. Despite having two developmental sources, cardiac fibroblasts are the most common fibroblast source.

A novel cell type that developed during the fibrotic response is the myofibroblast. Research findings imply that an active fibroblast can revert to a resting fibroblast. But concerns remain on the role of the fibroblast in physiology and illness.

Where are fibroblasts found in the skin?

The barrier system of the body includes the skin and its appendages, which account for 16% of body weight. The skin has an epidermis and dermis, with the dermis being the inner layer. Dermal fibroblasts are cells in the dermis that help the skin heal.

The skin protects the tissues beneath it from injury, infection, and water loss. Regulation of body temperature and reception of sensations are tasks of your skin. It also regulates sweat gland output and absorbs UV light to make vitamin D.

The dermis consists of a top papillary layer and a reticular layer that is denser and deeper in the skin. Collagenous connective tissues support the epidermis and connect the skin to the hypothalamus. In the eyelid, dermal thickness ranges from 0.6 to 3 mm.

The dermis-fascia interface is not well-defined. It is also thicker in the dorsal areas of the body. If you didn’t know, women have a thicker dermis than men.

In connective tissues, fibroblasts predominate. They help secrete extrarenal-matrix prophylactic material to keep connective tissues intact. All extracellular matrix molecules, including the primary material and strands, need precursors.

Fibroblasts, like other connective tissue cells, come from mesenchyme. The intermediate filament secretes vimentin, a protein used to identify mesodermal origins. Changing cells from one type to another happens in the epithelial-mesenchymal transition.

During certain circumstances, fibroblasts can turn into epithelial cells. Some fibroblasts make collagen, glycosaminoglycan, lattice, and elastic fibers. Glycoproteins in the EC and thymic stromal lymphopoietin cytokines are also included.

Unlike epithelium, fibroblasts are not restricted to the basal layer. They also help to build the basal layer. Because myofibroblasts make laminin chains, they don’t have follicular areas. Unlike epithelium, fibroblasts can move to the substrate layer on their own.

Fibroblasts can rebuild the structure of the skin. Injuries cause fibroblasts and mitosis to happen. These cells move to the wound, make ECM, and heal tissues during injuries. Proteins in the extracellular matrix help inflammatory cells move and form granular tissues.

Granular tissue growth allows keratinocytes to strengthen the epithelial tissues. The fibroblasts make contractile elements that help close the ulcer. Type I collagen also speeds up the healing process.

Collagen is the most common ECM component, and it makes fibers that make tissues look the way they do. Precursor collagen gets released by fibroblastic cells. When injected into the skin, you can use fibroblasts to synthesize new skin tissue.

How do fibroblasts heal wounds?

Fibroblasts are one of the most common cell types in connective tissues, and they make up a lot of them. They oversee keeping the body’s tissues in balance under normal conditions. They are also involved with wound healing.

In injury, fibroblasts become activated and become myofibroblasts. This event causes contractions and makes ECM proteins to help close the wound. Both fibroblasts and myofibroblasts generate contractile forces, which allow the injury to close.

Inflammation, proliferation, and remodeling are three typical stages of wound healing. It happens during the growth phase, which signals granulation tissue formation. Contraction is an integral part of healing a wound because it helps close it.

Wound contraction in humans can be both good and bad. It helps wound healing by reducing the size of the wound margins, leading to the wound’s closure. Yet, excessive contraction leads to contracture and scarring, which can cause problems.

Fibroblasts are a type of cell that can be non-contractile or very contractile. You can find them in most tissues, from mesenchymal cells. Fibroblasts help keep tissues healthy by controlling the turnover of ECM.

Studies show that both fibroblasts and myofibroblasts help in the healing process. The traction of fibroblasts and the coordinated contraction of myofibroblasts are vital factors. When too many myofibroblasts are active, scar tissue can form, resulting in immobilization.

What happens to fibroblasts as we age?

Aging is a part of human nature. As all living creatures do, humans deteriorate and die when cells stop regenerating. The same is true with fibroblasts, which undergo impaired metabolism and collagen production.

As you age, your body loses fibrous tissue and slows cell turnover. There is also damage to the water barrier function. Normal physiological skin functions may decline by 50% until middle age.

Progressive tissue decay and hormonal changes are causes of intrinsic aging. Metabolic reactions such as oxidative stress can also be its cause. The elastic fibers in the dermis deteriorate, causing skin atrophy and tiny wrinkles.

Meanwhile, the causes of extrinsic aging are too much sun exposure or smoking. A decrease in antioxidant capacity makes the skin more susceptible to sun damage. Aging also results in increased reactive oxygen species produced by skin cell metabolism.

These stressors affect a lot of different biochemical pathways. Effects include growth factor receptor-II deficiency and damage to the skin’s structural proteins. Furthermore, some studies show that both mechanisms of aging have areas that overlap.

For the skin, telomeres keep getting shorter, making it hard for cells to reproduce. The matrix and fibroblast pattern expression remains fixed in the dermis for a long time. When stimulation, the cells grow, but the telomeres stay the same length.

References:

de Araújo, R., Lôbo, M., Trindade, K., Silva, D., & Pereira, N. (2019). Fibroblast Growth Factors: A Controlling Mechanism of Skin Aging. Skin Pharmacology and Physiology, 32(5), 275–282.

Ivey, M. J., & Tallquist, M. D. (2016). Defining the Cardiac Fibroblast. Circulation Journal, 80(11), 2269–2276.

Kendall, R. T., & Feghali-Bostwick, C. A. (2014). Fibroblasts in fibrosis: novel roles and mediators. Frontiers in Pharmacology, 5.

Mandal, A. (2019, February 26). What are Fibroblasts? News-Medical

Nilforoushzadeh, M. A., Ahmadi Ashtiani, H. R., Jaffary, F., Jahangiri, F., Nikkhah, N., Mahmoudbeyk, M., Fard, M., Ansari, Z., & Zare, S. (2017). Dermal Fibroblast Cells: Biology and Function in Skin Regeneration. Journal of Skin and Stem Cell, In Press(In Press).

Sendic, G. (2022, March 16). Fibroblast. Kenhub.

Souders, C. A., Bowers, S. L. K., & Baudino, T. A. (2009). Cardiac Fibroblast. Circulation Research, 105(12), 1164–1176.

Wu, M., Cronin, K., & Crane, J. S. (2021, September 13). Biochemistry, Collagen Synthesis. Nih.gov; StatPearls Publishing. https://www.ncbi.nlm.nih.gov/books/NBK507709/#:~:text=The%20process%20of%20collagen% 20synthesis,occurs%20both%20intracellularly%20and%20extracellularly.