Components of the digestive system
Anatomically, the digestive system is divided into two main components: the digestive tube itself and the associated large glands.
Large glands lie outside the tract wall but secrete their products into its lumen for the chemical processing of food. These include:
- Major salivary glands (parotid, submandibular, sublingual);
- Liver (along with the biliary system);
- Pancreas (its exocrine part).
In the craniocaudal direction, the digestive tube includes the oral cavity, pharynx, esophagus, stomach, small intestine (duodenum, jejunum, ileum), and large intestine (cecum with appendix, colon, and rectum).
Classification into three regions
Based on embryonic development and functions, three regions of the digestive tube are distinguished:
- Foregut (oral cavity, pharynx, esophagus). Responsible for mechanical processing and transport of food.
- Midgut (stomach, small intestine, and most of the large intestine, as well as the liver and pancreas). Provides the main chemical processing, absorption of nutrients, and formation of feces.
- Hindgut (caudal part of the rectum). Performs the function of evacuating undigested residues.
Wall structure: layers
The wall of the hollow tract consists of four main layers (from inside out):
- Mucous membrane (Tunica mucosa). Moistened by mucus. To increase the absorption surface area, it can form folds, pits, crypts, and villi. It consists of epithelium (squamous or columnar depending on the region), lamina propria (loose connective tissue), and muscularis mucosae.
- Submucosa (Tela submucosa). Formed by a thick layer of loose connective tissue containing large blood vessels and nerve plexuses. It allows the mucosa to gather into folds. Absent on the upper surface of the tongue, gums, and hard palate.
- Muscular layer (Tunica muscularis). Includes an inner circular and an outer longitudinal layer (a third, inner oblique layer is added in the stomach). In the pharynx, the beginning of the esophagus, and the end of the rectum, it is formed by striated muscle tissue; in other regions, it is smooth.
- Outer layer. Of two types: adventitial (connective tissue only, in the pharynx, thoracic esophagus, and terminal rectum) or serous (visceral peritoneum covered by mesothelium, in abdominal regions).
Epithelium of the digestive tube
The type of epithelium is closely related to the function of the region:
- In the foregut (protection against mechanical damage) — stratified squamous.
- In the midgut (absorption and secretion) — simple columnar (prismatic). The apical surface of small intestine cells bears microvilli forming the brush border.
- In the hindgut (terminal part of the rectum) — once again stratified, gradually transitioning from cuboidal to keratinized squamous.
Briefly: answers to topical questions
What types of cells are part of the epithelium of the small intestinal mucosa?
The epithelium of the small intestinal mucosa includes five types of cells.
- Columnar epithelial cells (enterocytes, brush border cells) — the predominant cells carrying out absorption, including a subtype of M-cells.
- Goblet cells — unicellular glands that secrete mucus.
- Endocrinocytes — cells providing hormonal regulation.
- Exocrinocytes with acidophilic granules (Paneth cells) — located at the base of the crypts.
- Undifferentiated epithelial cells — cambial stem cells.
The crypt epithelium contains all five types, whereas the villus epithelium features only the first three cell types, which have migrated from the crypts.
What intramural nerve plexuses are located in the wall of the digestive tract?
Three types of intramural nerve plexuses are located in the wall of the digestive tube.
- Submucosal plexus (Meissner's) — located in the submucosa.
- Myenteric plexus (Auerbach's) — located between the layers of the muscular coat.
- Subserous / subadventitial plexus — localized at the boundary between the muscular layer and the outer layer.
In the esophagus, an additional adventitial plexus is distinguished. Intramural ganglia contain cell bodies of sensory, associative, and effector neurons; all three cell types are represented in the enteric (metasympathetic) nervous system, whereas only effector neurons are present in the parasympathetic system.
From which germ layers does the epithelium of various regions of the digestive tube develop?
The epithelial lining of various regions of the digestive tube develops from the ectoderm and endoderm.
- Foregut is described as a derivative of mixed ecto-endodermal material due to the region of the oropharyngeal membrane; however, the oral epithelium is of ectodermal origin, while the esophageal epithelium is of endodermal origin (subject to ongoing debate).
- Midgut — derivative of the endoderm: epithelium of the stomach, small intestine, and large intestine.
- Hindgut — derivative of the ectoderm: epithelium of the terminal, anal part of the rectum.
To remember the layers of the GI tract wall from inside out: use the Russian mnemonic.
Class and exam questions
In which parts of the gastrointestinal tract is the muscular tunic formed by striated muscle tissue?
In the pharynx, esophagus (up to the middle third inclusively), and the caudal part of the rectum.
What is the difference between the serous membrane and the adventitia?
Unlike the adventitia, the serous membrane is covered externally by a single layer of flat cells — the mesothelium. The adventitia consists exclusively of connective tissue.
Where in the digestive tube is the muscular plate of the mucosa absent?
It is absent in the oral cavity and pharynx, appearing only starting from the upper third of the esophagus.
What else to study on this topic
- Features of mucosal relief: pits, villi, crypts
- Intramural nervous apparatus and types of plexuses
- Secretory activity of the epithelium (exocrinocytes and endocrinocytes)
- Localization of glands in the lamina propria and submucosa
- Structural features of the oral cavity due to the absence of a submucosa
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