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”.

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

More features and more interactions in the Getaprofessor app. Whether you are from Silliman University or from other universities, you are invited to our app launching this Aug.19 at Silliman University, Mariano Lao bldg., Activity Center. Entrepreneurs, startup enablers, students, educators and anyone interested to learn more about our app is invited to join. See you there!

Surprises await you!

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.

Mucosa

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.

Submucosa

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.

REFERENCES:

Bailey, R. (2019). Anatomy of the Stomach. Retrieved from https://www.thoughtco.com/anatomy-of-the-stomach-373482

Cleveland            Clinic            (2021).            Small            Intestine.            Retrieved            from https://my.clevelandclinic.org/health/body/22135-small-intestine

Histology at SIU (2020). Study Guide Histology of the Gastrointestinal System. Retrieved from https://histology.siu.edu/erg/giguide.htm#submucosa

Histology Guide, (n.d.). Chapter 14 – Gastrointestinal Tract. Retrieved from https://histologyguide.com/slidebox/14-gastrointestinal-tract.html

Lapid, N. (2020). How the Intestinal Villi Help With Digestion. Retrieved from https://www.verywellhealth.com/understanding-intestinal-villi-562555

Mescher, A. (2018). Junqueira’s Basic Histology Text and Atlas. (15th Ed). McGraw-Hill Education.

NIH (n.d.). General Structure of the Digestive System. Retrieved from https://training.seer.cancer.gov/anatomy/digestive/structure.html

Pirie,              E.             (2022).             Stomach             histology.             Retrieved             from https://www.kenhub.com/en/library/anatomy/stomach-histology

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. https://www.britannica.com/science/human-digestive-system

Socratic (n.d.). Villi in the Small Intestine. Retrieved from https://socratic.org/biology/input-and-output/villi-in-the-small-intestine

University of Rochester Medical Center (n.d.). Anatomy of the Stomach. Retrieved from https://www.urmc.rochester.edu/encyclopedia/content.aspx?contenttypeid=34&contentid=17785- 1#:~:text=for%20the%20serosa.-,Serosa.,the%20stomach%20to%20confine%20it.

Walker,              D.              (2021).              The              Stomach.              Retrieved              from https://teachmeanatomy.info/abdomen/gi-tract/stomach/

Written by Angelyn Evan S. Bomediano

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.

References

Britannica, T. Editors of Encyclopaedia (2020, February 28). keratin. Encyclopedia Britannica. https://www.britannica.com/science/keratin

Cozzani E, Nicolas JF, Reano A, Schmitt D. Le desmosome: structure, fonction et pathologie acquise [The desmosome: structure, function and acquired pathology]. Pathol Biol (Paris). 1994 Jun;42(6):603-11. French. PMID: 7854861

Gröne A. Keratinocytes and cytokines. Vet Immunol Immunopathol. 2002 Sep 6;88(1-2):1-12.

10.1016/s0165-2427(02)00136-8. PMID: 12088639

Pasch MC, Van Den Bosch NH, Daha MR, Bos JD, Asghar SS. Synthesis of complement components C3 and factor B in human keratinocytes is differentially regulated by cytokines. J Invest Dermatol. 2000 Jan;114(1):78-82. 10.1046/j.1523- 1747.2000.00841.x. PMID: 10620119

Shirakata Y. Regulation of epidermal keratinocytes by growth factors. J Dermatol Sci. 2010 Aug;59(2):73-80. 10.1016/j.jdermsci.2010.05.002. Epub 2010 May 19. PMID: 20570492

Szabó G, Gerald AB, Pathak MA, Fitzpatrick TB. Racial differences in the fate of melanosomes in human epidermis. Nature. 1969;222(5198):1081–2

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

Yen TT, Thao DT, Thuoc TL. An overview on keratinocyte growth factor: from the molecular properties to clinical applications. Protein Pept Lett. 2014 Mar;21(3):306-17.

10.2174/09298665113206660115. PMID: 24188496

George, B. (2017). INTEGUMENTARY SYSTEM Bio 137 Anatomy & Physiology I [PowerPoint slides]. Slide Player. https://slideplayer.com/slide/9442336/

Helmenstine, A. (2016, January 8). What is keratin and what is its

purpose? ThoughtCo. https://www.thoughtco.com/keratin-definition-and-purpose- 608202

Keratinocytes. (2019, December 10). PromoCell. https://promocell.com/cell-culture- basics/keratinocytes/

Lakna. (2018, January 22). Difference between Keratinocytes and melanocytes. Pediaa.Com. https://pediaa.com/difference-between-keratinocytes-and-

melanocytes/#:~:text=The%20main%20difference%20between%20keratinocytes,from% 20UV%20by%20producing%20melanin

Reina, O. (2016, September 13). Keratinocytes: Their purpose, their subtypes and their lifecycle. Tempo Bioscience. https://www.tempobioscience.com/blog/keratinocytes- their-purpose-their-subtypes-and-their-lifecycle/

Rios, A. (2013). Cytology in Skin Diseases.

VetFolio. https://www.vetfolio.com/learn/article/cytology-in-skin-diseases W, B., & Robb. (2022, April 2). What is Keratin? Structure and Function.

Study.com. https://study.com/learn/lesson/keratin-overview-structure-function.html Watson, K. (2018, September 17). Keratin: Benefits and uses for hair.

Healthline. https://www.healthline.com/health/keratin

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.

References:

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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:

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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

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