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How my mama made eternity real for me

Losing my mom is one of the deepest pain that I have ever experienced in my life, but it also made me realize that eternity is real. The last days of mama’s life made such a life-changing impression on me. What I experienced when I entered the ICU room of Holy Child Hospital in Dumaguete City on August 25, 2022, will stay with me for the rest of my life. Her eyes had tears when she took her last breath on this earth. I could feel heaven right there in the room. It was almost as if I could see Jesus Christ coming to take her to heaven, and I also had a great desire to go there.

I can never describe the feeling of peace that filled the ICU room at that moment, and in those moments, I made a decision that I would live the rest of my days here on earth in such a way that I would see Jesus Christ and my mom when my time comes. It would be foolish to become burdened by things of this earth. I am reminded by this verse from Matthew 6:20-21 “But lay up for yourselves treasures in heaven, where neither moth nor rust destroys and where thieves do not break in and steal. For where your treasure is, there your heart will be also”. Our possessions and achievements in this earthly world would still pass away. It is better to store up treasures that can last for eternity, and for me, that would be knowing more about Jesus Christ, His ministry, and having a personal relationship with Him.

As the apostle Paul stated in Philippians 1:21, ” to live is Christ, to die is gain” he understood the sufferings we have in this world. It was better to die and be with Christ forever, but it is not for us to decide when that would be, and so we continue living for Christ here on earth. Fighting against our sins that continue to ensnare us while looking to Christ to deliver us from our transgressions.

Even though I am experiencing grief after losing mama, it makes heaven closer when I think about her and Jesus Christ on the other side. I look forward to meeting my mama again one day in heaven. The tremendous peace that I witnessed and experienced that fateful evening on August 25, 2022, when Jesus came to take my mama home, has given me a glimpse of what awaits us in eternity. So I continue to live my life for Christ and strive to gain victory over sins that would hinder me in my walk with the Lord Jesus Christ.

I am also reminded by this verse from 2 Corinthians 4:16-18 ” Therefore we do not lose heart. Even though our outward man is perishing, yet the inward man is being renewed day by day. For our light affliction, which is but for a moment, is working for us a far more exceeding and eternal weight of glory, while we do not look at the things which are seen, but at the things which are not seen. For the things which are seen are temporary, but the things which are not seen are eternal”.

Philippians 3:20 ” For our citizenship is in heaven, from which we also eagerly wait for the Savior, the Lord Jesus Christ.”

Reuben J Chavez Los Baños

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What are the 4 major histological layers of the digestive system?

Written by Ayessa G. Ibañez

What are the 4 major layers of the digestive system? The gastrointestinal tract or GI tract makes up most of our digestive system. All parts in the system have common structural features to fulfill their role. These are the mucosa, submucosa, muscular, and the serous layer, the four major layers.

What are the 4 major histological layers of the digestive system?


The mucosa is also known as the mucous membrane layer. You can find this layer in the innermost tunic of the wall. Moreover, it lines the lumen of the digestive tract.

This histological layer of the digestive system has varied different tissues present. It consists of an epithelial lining, lamina propria, and muscularis mucosae.

The lining epithelium varies in the layer on a certain part. For example, the mucosal layer of the esophagus has a stratified squamous epithelium. The simple columnar epithelium is in the gastrointestinal part of the alimentary tract.

The lamina propria is an underlying loose connective tissue. It is rich in blood vessels and lymphatics. It also has lymphocytes, smooth muscle cells, and often small glands.

Consisting of smooth muscle, the muscularis mucosa separates the mucosa from the submucosa. It separates mucosa from submucosa, giving the former local action.

With these structural components of the mucosa, it can do its function. Protect, absorb, and secrete. These are the three principal functions of the mucous membrane layer.

The epithelial lining serves as a barrier. It separates the lumen of the alimentary canal from the external luminal environment. Through this barrier, we have protection from antigens, pathogens, and other toxic substances.

For example, the mucosa of the esophagus has a stratified squamous epithelium. The esophagus has protection from physical abrasion when ingesting food through this epithelium.

The mucosa has surface projections like the villi and microvilli. It increases the surface area available for its absorptive function.

As for its secretory function, the mucous membrane layer has mucosal glands. It provides mucus for protective lubrication and substances that aid in digestion.


Surrounding and protecting the mucosa is the submucosa layer. It consists of a thick layer of dense irregular connective tissue. This type of tissue allows the mucosa to move during peristalsis in a flexible manner.

There are also larger blood vessels, lymphatic vessels, and a nerve plexus in this layer. The vascular plexus, large veins, and arteries give rise to the capillary bed of the mucosa.

A delicate nerve network makes up the nerve plexus called Meissner’s plexus. Also termed the submucosal plexus, it has unmyelinated nerve fibers and ganglion cells.

Glands can be present in the submucosa in some areas, referred to as the submucosal glands. The esophagus has occasional submucosal mucous glands. Submucosal glands are also in the duodenum, packed with mucous Brunner’s glands.

Muscularis externa

The muscular layer is also called muscularis externa or “muscularis” for short. This third layer is the muscular wall of the GI tract, deep into and surrounding the submucosa.

The layer has two concentric and thick layers of smooth muscle. A circularly oriented layer is the inner layer that has cells forming a tight spiral. Forming a loose spiral in the outer layer makes up the longitudinally oriented layer.

Found between the two muscle layers is a thin connective tissue layer. Within this part is the location of the myenteric plexus or the Auerbach’s plexus. It has postganglionic parasympathetic neurons and the neurons of the enteric nervous system.

The smooth muscle characteristic of muscularis aids movements of the digestive tract. The inner and outer circular layer contracts, allowing compression and propelling. It creates a slow, rhythmic contraction causing peristalsis or waves of contraction.

Serous layer

The superficial layer of the digestive tract is the serous layer, or serosa, or adventitia. It is a serous membrane made up of simple squamous epithelium called the mesothelium. It also has a small part of underlying connective tissue.

The varying names are dependent on the location and function of structures.

It holds the term adventitia when the outermost layer attaches to surrounding tissue. It has ordinary fibrous connective tissue arranged around the organ that it supports. The role of adventitia is to hold the internal structures together.

The term serosa is when the outermost layer lies next to the peritoneal cavity. It consists of ordinary connective tissue with a surface of the mesothelium. The serosa functions to lubricate the internal structures of the body.

What type of epithelium is in the digestive tract?

The inner surface of the organs of the digestive system has epithelial coverings. These epithelial tissues are simple columnar and stratified squamous epithelium.

The mucosa of the tongue and esophagus has stratified squamous non-keratinized epithelium. Moreover, the submucosa in the esophagus usually has cuboidal epithelium ducts.

The simple columnar epithelium is present in the stomach and small intestine. But, only small intestines have the columnar epithelium that has microvilli and villi.

What is unique about the histology of the stomach?

The stomach shares the same common histological layers as the rest of the GI tract. But, its unique feature lies in the fact that it contains many microscopic glands. These glands secrete substances needed by the stomach to serve its function.

Among the glands, the most essential are the glands in the cardiac, pyloric, and fundic regions.

The cardiac glands have mucus-secreting cells. Its secretion contributes to gastric juice. It also helps protect the esophageal epithelium against acid reflux.

Like the cardiac gland, the pyloric glands secrete mucus, which coats the stomach. This protects the stomach from self-digestion by helping to dilute acids and enzymes.

The fundic glands are responsible for producing gastric juices in the stomach.

What types of cells are in the stomach?

The stomach has a simple columnar type of epithelium. This type of epithelium holds many tubular gastric glands. The glands, also referred to as the fundic glands, produce the stomach’s gastric juice.

The gastric glands in the stomach’s mucosal lining have four different cell types. These are the mucous, parietal, chief, and endocrine cells. Each cell has a distinctive characteristic and function.

Mucous cells

Mucous cells are common to all types of gastric glands. They are the primary cell type found in the gastric glands. These cells are also present in cardiac and pyloric areas of the stomach.

The neck of the fundic glands of the stomach has mucoid cells in its lining.

This type of cell secretes an alkaline mucus. It protects the epithelium against shear stress and acid.

Parietal cells

Also called oxyntic cells, parietal cells are in the neck and deeper part of the fundic glands. They are large cells with spherical nuclei, appearing to have a triangle shape.

It secretes hydrochloric acid (HCl) from the combination of hydrogen and chloride ions. The produced acid moves into the gland’s lumen and then passes through to the stomach.

Chief cells

Chief cells or zymogenic cells are typical protein-secreting cells. This type of cell is also found in the deeper part of the gastric gland.

The pepsinogen in the stomach is from the chief cells of the gland. The cells secrete pepsinogen, which converts upon contact with gastric juices. Pepsinogen becomes a proteolytic enzyme called pepsin.

Endocrine cells

Endocrin cells, called enterochromaffin-like cells, scatters throughout the body of the stomach. Enterochromaffin-like cells secrete hormones based on the information from the chemoreceptors. It includes the secretion of the hormone gastrin.

What is the outer layer of the stomach called?

Our stomach has five layers. Like every organ in the GI tract, it has the mucosa, submucosa, muscularis, subserosa, and the serosa. As already arranged in sequence, the stomach’s outer layer is the serosa.

We can call the serosa of the stomach as gastric serosa. Like the general serosa, it comprises simple squamous epithelium or the mesothelium. Moreover, it has a thin layer of underlying connective tissue.

Through its serous-secreting mesothelium, the serosa lubricates the outer wall of the stomach. The serous fluid ensures smooth movement in the abdominal cavity. There will be less friction with smooth movements as the GI tract organs work.

What is the greater curvature of the stomach?

Your stomach looks like a J-shaped organ. If you divide it into half, the organ’s right side curve or the outside curve is the greater curvature. The hollow curve on the left side is the lesser curvature, parallel to the greater curvature.

The curvatures look like two Cs in the lateral inversion or two close parentheses. )). The first one is the lesser curvature, while the latter is the greater curvature.

Well, that is an easy way of identifying the curvatures in Layman’s term.

In technical terms, the greater curvature is a long, convex, lateral border of the stomach. Arising first at the cardiac notch, it arches backward and passes inferior to the left. It curves to the right and continues in the medial to reach the pyloric antrum.

The curvatures have a structural association with the blood supply of the stomach. The greater curvature has blood supply through the short gastric arteries. Another supply branch of this curvature is the right and left gastro-omental arteries.

What are the 3 divisions of the small intestine?

The digestion process completes in the small intestine. After breaking down the foods, 90% of nutrient absorption occurs in this structure. Thus, it is the most crucial absorbing organ in the GI tract.

The small intestine has three divisions or segments to aid its digestion role. These are the duodenum, jejunum, and ileum, making up the long structure of the small intestine.

The three segments have histological features that they share in common. They all have the four primary histological layers of the GI tract. But, the jejunum and ileum have more similarities.

They all have the mucosa that has villi with enterocytes and goblet cells. Also, they have crypts, intestinal glands at the base of the villi, and the muscularis mucosae. But the jejunum and ileum have paneth cells and stem cells in the crypts.

The three segments have submucosa, but only the duodenum has a submucosal gland. The gland in the duodenum are Brunner’s glands. Moreover, only the jejunum and ileum have the submucosal plexus or Meissner’s Plexus.

These structures aid the small intestine’s function.

The first part of the small intestine is the duodenum. It signals other digestive organs to release chemicals when there is food. These chemicals are digestive juices that help break food down.

The jejunum is where the digested food from the duodenum comes next. The muscles in the intestinal walls churn food back and forth. It allows the food to mix with digestive juices and keep moving forward.

The final and extensive part of the small intestine is the ileum. Its function is to digest the food further. It will absorb any remaining nutrients that did not get absorbed from the first segments.

What is the importance of the villi in the small intestine?

Villi are short mucosal outgrowths that cover the mucosa of the small intestine. They appear as finger- or leaflike projections. Each projection has a covering of simple columnar epithelium absorptive cells.

The absorptive cells are also termed enterocytes. They are tall columnar cells with an oval nucleus at the basal part.

Your villi are vital features of the small intestine. Through these structures, you can get the main aim of why we intake food. To get nutrients that our body needs.

Villi absorb nutrients and complete the breakdown of food.

Without functional intestinal villi, you won’t get any nutrients from your food. Even though you will eat a lot, your body will not absorb and use the food. You will end up malnourishment or starvation.

The design of its structure enables its function.

A villus (singular of villi) has a large surface area. It increases the mucosal surface area when in contact with nutrients. With a larger surface area, there is also a larger absorptive area to absorb the nutrients from the food.

Moreover, the villi have a thin epithelium wall. This characteristic reduces the distance that materials need to move. It also increases the diffusion rate of nutrients into the blood for delivery to cells.

Also assisting the transport of nutrients is the villi’s moist feature.

The nutrient absorption mechanism may also vary depending on the other structures. The lacteal or tiny lymphatic vessels in a villus absorb fatty acids and glycerol. There are also blood capillaries that absorb glucose and amino acids.


Bailey, R. (2019). Anatomy of the Stomach. Retrieved from

Cleveland            Clinic            (2021).            Small            Intestine.            Retrieved            from

Histology at SIU (2020). Study Guide Histology of the Gastrointestinal System. Retrieved from

Histology Guide, (n.d.). Chapter 14 – Gastrointestinal Tract. Retrieved from

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Ross, M. (2016). Histology: A Text and Atlas: With Correlated Cell and Molecular Biology. (7th Ed.). Wolters Kluwer Health.

Sircus, W. , Dworken, . Harvey J. , Hightower, . Nicholas Carr and Keeton, . William T. (2020, November 4). human digestive system. Encyclopedia Britannica.

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University of Rochester Medical Center (n.d.). Anatomy of the Stomach. Retrieved from 1#:~:text=for%20the%20serosa.-,Serosa.,the%20stomach%20to%20confine%20it.

Walker,              D.              (2021).              The              Stomach.              Retrieved              from

What are keratinocytes and keratin?

Written by Angelyn Evan S. Bomediano

What are keratinocytes and keratin?

I.   Keratinocytes (KC)

A. Definition

Keratinocytes are the typical type of cell in the epidermis, the top layer of skin. They make up about 90% of the cells in the epidermis. They start in the stratum basale, the deepest layer, and move up to the stratum corneum, the outermost part. Also, they are flat, squamous cells with a lot of keratin but no nucleus.

They accumulate in the basal layer and change as they move toward the skin’s surface. This process is gradual differentiation. It changes shape and starts making keratin, cytokines, growth factors, interleukins, and complement factors. Moreover, it is under control by several factors and mechanisms that work on the epigenome.

B.  Function

Keratinocytes have a particular job to do. They are essential for protection and this is due to their formation, a tight barrier that keeps foreign substances from getting into the body and keeps moisture, heat, and other vital things from escaping.

These cells also have a structural role, forming tight bonds with the other cells in the epidermis and maintaining them in their locations. KC also acts as an immune system regulator when the skin hurts.

C.  Applications

  1. Keratinocyte Cell Culture

Rheinwald and Green were the first people 30 years ago to write about developing human KC in a single layer. Since then, there have been many improvements in how human keratinocytes are grown. They can now be in both two-dimensional and three-dimensional cultures.

Normal Human Epidermal Keratinocytes (NHEK) can be from a young person’s foreskin or normal human tissue on an adult’s face, breast, abdomen, or thighs, among other places. NHEK from a single donor or a group of donors can be for research uses. The right amount of calcium in the culture is vital to ensure that KC can grow and change in the best way.

2.  Characterization

Keratinocytes express specific keratins at each stage of differentiation and other markers like involucrin, loricrin, transglutaminase, filaggrin, and caspase 14.

3.  Applications of Research

KC are available for many things, like studying how the epidermis grows and changes, how the body take in drugs, testing cosmetics and toxins, and analyzing how integument ages. They are also available in skin research, how wounds heal, and cancer research.

D.  Interactions with other Cells in the Skin

  1. Keratinocytes and Fibroblasts

Cross-talk between KC and fibroblasts is essential for keeping the skin balanced and healing wounds. Paracrine signaling is how these two types of cells interact with each other. Interrupting this cross-talk can cause chronic injuries.

2.  Keratinocytes and Melanocytes

KC and melanocytes need to communicate for the epidermis to stay in balance. Melanocytes make melanin, which soaks up UV waves and keeps KC’s DNA from getting damaged. On the other hand, KC helps melanocytes multiply, change, and make more melanin.

3.  Keratinocytes and other cells

KC works with lymphocytes and cellular Langerhans in the integument to change how the immune system works.

E.  Role in wound healing and inflammation

  1. Keratinocytes in Wound Healing

KC is in charge of repairing damage to the epidermis. It is called “re-epithelialization,” and a wound must heal. When the skin undergoes stress, KC becomes active, moves to the injury, and starts making more of themselves to fill the hole.

When an injury is healing, KC, fibroblasts, and immune cells need to communicate. When KC doesn’t work right, wounds don’t heal and stay open for a long time.

2.  Keratinocytes in Inflammation

When the epidermal barrier breaks or pathogens get into the skin, the body responds with inflammation. Keratinocytes participate in this process because they make cytokines, which send signals to immune cells that can be good or bad.

Keratinocytes also play a role in several allergic skins diseases and chronic inflammatory conditions like psoriasis because they can recruit and activate dendritic cells and leukocytes.

II.   Keratin (K)

  1. Definition

Keratin is a type of protein seen in hair, skin, and nails. K is a potent, fibrous protein that can’t be scratched or torn. It is also called an intermediate filament, an essential protein that gives the structure of hair, skin, and nails.

Like other proteins, keratin rises from amino acids. Each protein possesses its own set of amino acids, like each person has deoxyribonucleic acid (DNA). Though keratin can be in organs and glands, it is in cells that line the inside of the body. Endothelial cells cover the body’s surface.

K also includes the digestive and urinary tracts, both inside the body but opening to the outside world. K can withstand different environmental conditions because it is strong and has a solid structure.

B.  Location

Keratin is in the top layer of skin, called the epidermis. The integument is the biggest organ in the body, and it protects the organs inside it. New skin comes from the bottom of the epidermis while the old ones rise to the top and fall off. What’s left of dead skin tissue is usually a mix of proteins, with keratin being the main one. In this way, most surface-level parts of the epidermis are composed of the keratin epidermis’s keratin cells.

C.  Types of Keratin

There are 54 types of keratin protein in the body. The main four types are:

  • Type I – The proteins in type I K are usually small and acidic. Acidic molecule can either give another molecule a proton (hydrogen ion) or form a covalent bond with an electron pair. Covalent bonds happen when two molecules share electrons. This type is significant to the health of epithelial cells.
  • Type II – Keratins of type II are large proteins that have a pH of 7.
  • Alpha-keratin – Alpha-keratins are in humans and the wools of other mammals. Since they are fibrous and helical, this type helps keep the structure of epithelia cells strong.
  • Beta-keratin – Only birds and reptiles have beta-keratins. They help to keep the shape of:
    • Claws
    • Scales
    • Beaks
    • Skin
    • Feathers

D.  Keratin Structure

The structure and function of keratin depend on what amino acids are in it. The shape of a protein molecule depends on the kinds of amino acids and how they connect. The same protein molecule can have more than one structure:

Primary structure: The order of amino acids within one protein molecule

Secondary structure: Amino acids bond together to form an alpha-helix (coil shape) or beta- pleated sheet (accordion shape)

Tertiary structure: One port of a protein chain binds with another part of the same chain

Quarternary structure: A complex protein structure where more than one protein chain binds to another

Due to its robust and stable structure, Keratin is insoluble in water and cannot affect acids, alkalis, or other powerful solvents. As a result, keratin can withstand circumstances both within and outside the human body. Keratin shrinks when exposed to water at high temperatures because some bonds break because to the high heat.

E.  Function

Keratin controls epithelial cells’ formation and protection, strengthening the skin and supporting internal organs. It also keeps the skin elastic and preserves the skin’s suppleness. Additionally, it binds epithelial cells together and assists them in resisting the effects of mechanical stress.

What do keratinocytes contain?

KC develops in the basal layer of the epidermis and differentiates as they rise. During this process, they change their shape and start to produce keratin, cytokines, growth factors, interleukins, and complement factors. Several factors and epigenetic pathways influence KC differentiation.

A.  Keratin

KC produces keratin that makes up most of the structure of the skin, hair, and nails.

This component has an essential role in mediating cutaneous immune responses, inflammation, wound healing, and the growth and development of certain neoplasms.

B.  Cytokines

KC is known to produce cytokines as well. That said, cytokines regulate immune and inflammatory responses and play essential roles in pathological skin conditions.

C.  Growth Factors

KGF (Keratinocyte Growth Factor), also known as FGF7, promotes the migration and differentiation of epithelial cells and protects them against stress. KGF is generated by mesenchymal cells and exerts its biological effects by binding to its high-affinity receptor, a splice variant of FGF receptor 2 (FGFR2-IIIb), expressed by epithelial cells, including epidermal keratinocytes.

D.  Interleukins

Despite being given less attention, keratinocytes also produce a variety of cytokines, including interleukin (IL)-1, -6, -7, -8, -10, -12, -15, -18, and -20, as well as tumor necrosis factor-alpha (TNF). Wherein (IL)-1, -6, -8, and TNF were discovered and investigated.

E.  Complement Factors

KC is the predominant cell type in the skin; this cell type produces two soluble components of the complement system, C3 and factor B.

F.  Others

Also produce eicosanoids, prostaglandin (PG) E2, and neuropeptides such as proopiomelanocortin and α MSH.

What is the difference between keratinocytes and melanocytes?

Keratinocytes refer to the epidermal cells that produce keratin, while Melanocytes refer to the mature melanin-forming cells in the skin. In terms of its differentiation, KC is from the basal layer of the epithelium.

At the same time, Melanocytes are from the neural crest cells. With regards to their production, KC is more on the display of Keratin which most of the epidermal cells are, while Melanocytes are on the production of Melanin, and it is lesser compared to the keratinocytes.

Moreover, KC is a physical barrier between organisms and the external environment and forms hair and nails. While the other is responsible for the color of the skin.

Where are the oldest keratinocytes in your skin found?

The oldest KC in the skin is at the outermost epidermal layers, the Stratum corneum. This comprises around 25-50 layers of KC filled with keratin continuously shedding off. Furthermore, the constant exertion of friction stimulates cell production in this layer and the production of callus.

Do keratinocytes produce vitamin D?

Yes, KC produces vitamin D. Besides that, they contain enzymatic machinery to convert vitamin D to active metabolites 1,25(OH)2D. In particular, the vitamin D receptor (VDR) allows the keratinocytes to respond to the 1,25(OH)2D.

Vitamin D and its receptors influence various skin activities such as inhibition of proliferation, encouragement of differentiation, including the creation of the permeability barrier, enhancement of innate immunity, hair follicle cycle modulation, and tumor suppression.

Do keratinocytes produce melanin?

Currently, some researchers discovered that melanin undergoes concentration in keratinocytes in the stratum basale, the deepest layer of integument and that the number of melanin granules correlates with complexion. Their findings revealed how melanin scatters within KC in different integument phototypes.

Previous research also revealed that melanosomes in dark skin occur as single membrane- delimited structures. In contrast, melanosomes in light skin only exist as clusters surrounded by a membrane.

What holds keratinocytes together in the epidermis?

The structures of resistance in the epidermis are the desmosomes, which allow the keratinocytes to stick to each other. The function of sticking together is possible by how the desmosomal molecules and cytoskeletal filaments work together. Moreover, whenever the skin undergoes physical trauma and rubbing, the desmosomes, connecting junctions in-between, help keep the cells together.

What are nucleated keratinocytes?

Normal KC is usually flat, keratinized squamous, and nucleus-free. With that, nucleated ones are known to be abnormal. However, it occurs seldomly. Moreover, it includes inflammatory cells and indicates parakeratosis, erosions, or that the sample was taken from below the stratum corneum.


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Timár KK, Pasch MC, van den Bosch NH, Jarva H, Junnikkala S, Meri S, Bos JD, Asghar SS. Human keratinocytes produce the complement inhibitor factor H: synthesis is regulated by interferon-gamma. Mol Immunol. 2006 Feb;43(4):317-25.

10.1016/j.molimm.2005.02.009. Epub 2005 Mar 16. PMID: 16310045

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10.2174/09298665113206660115. PMID: 24188496

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purpose? ThoughtCo. 608202

Keratinocytes. (2019, December 10). PromoCell. basics/keratinocytes/

Lakna. (2018, January 22). Difference between Keratinocytes and melanocytes. Pediaa.Com.

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Reina, O. (2016, September 13). Keratinocytes: Their purpose, their subtypes and their lifecycle. Tempo Bioscience. their-purpose-their-subtypes-and-their-lifecycle/

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VetFolio. W, B., & Robb. (2022, April 2). What is Keratin? Structure and Function. Watson, K. (2018, September 17). Keratin: Benefits and uses for hair.


What are the three histological layers of the heart?

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.

What are the three histological layers of the heart?

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.


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.


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.


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.


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