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/