Morphological transition from stomach to intestine
A histological specimen of the junction between the pyloric stomach and the duodenum clearly demonstrates the change in relief and wall layers. In the stomach, the mucosa forms deep gastric pits, while the muscularis mucosae and muscularis propria consist of three layers. The gastric submucosa lacks glands (pyloric mucous glands lie within the lamina propria).
Upon transition to the duodenum, the picture changes dramatically:
- Wide and short intestinal villi appear, as well as depressions—crypts (crypts of Lieberkühn).
- The number of muscle layers in the muscularis mucosae and muscularis propria decreases to two.
- Duodenal (Brunner's) glands appear in the submucosa, which is the main diagnostic feature of the duodenum.
- The longitudinal folds of the pylorus give way to longitudinal and then circular folds of the intestine.
- Lymphoid follicles are found more frequently in the duodenum than in the stomach.
Paneth cells (exocrinocytes with acidophilic granules)
Paneth cells are specialized exocrine cells located exclusively at the base of the crypts and are never found on the villi. When stained with hematoxylin and eosin, dense acidophilic granules staining intense pink or red are clearly visible in their apical part.
These cells perform two critical functions:
- Antimicrobial (protective) function: they secrete the enzyme lysozyme, which can destroy the cell walls of pathogenic bacteria and protozoa.
- Digestive function: they release dipeptidases—enzymes that break down dipeptides into free amino acids.
Endocrine apparatus of the small intestine
Endocrine cells (apudocytes) are scattered throughout the epithelium of both crypts (where their density is maximal) and villi. Unlike exocrinocytes, their hormone-containing secretory granules accumulate in the basal part of the cell. Hormones are released either into blood capillaries or paracrinically (affecting neighboring cells). A total of 6 types of such cells are identified in the intestine, 4 of which are also present in the stomach.
Main types of endocrinocytes:
- EC cells: the most numerous. Respond to the entry of chyme from the intestinal lumen. Produce serotonin and motilin, which stimulate evacuation function (motility) and enhance secretion of intestinal juice.
- S cells: activated upon the arrival of chyme. They secrete secretin, which "turns off" (inhibits) gastric secretion and "turns on" the release of bile and pancreatic juice.
- I cells: secrete cholecystokinin (pancreozymin). They act synergistically with S cells, powerfully stimulating biliary and pancreatic secretion.
Epithelial regeneration
Continuous physiological renewal of the epithelial layer is a critical process for gastrointestinal tract function. In the small intestine, the function of stem cells (cambium) is performed by undifferentiated epithelial cells located in the lower half of the crypts. By comparison, in the stomach, stem cells are located in the neck of the glands, i.e., closer to the surface.
Cells divide at the bottom of the crypt and begin migrating upward toward the tip of the villus. Upon reaching the tip, old epithelial cells are shed into the intestinal lumen. This entire life cycle takes about 5–6 days. The only exception to this rule is Paneth cells: after differentiation, they do not migrate upward but remain at the base of the crypts.
Differential diagnostics of small intestine segments
The duodenum has a number of morphological differences from the jejunum and ileum:
- Submucosa: Brunner's glands are present exclusively in the duodenum.
- Villi: in the duodenum, they are wide and short, whereas in the jejunum and ileum, they are thin and tall.
- Folds: in the jejunum, circular folds are frequent and tall, while in the ileum, they gradually decrease and disappear.
- Cellular composition: from the duodenum to the ileum, the number of goblet cells in the epithelium increases.
- Lymphoid tissue: solitary follicles are present everywhere, but large aggregates—Peyer's patches—form exclusively in the ileum.
Briefly: answers to topical questions
What types of endocrine cells (apudocytes) are present in the epithelium of the small intestine?
The epithelium of the small intestine contains 6 types of endocrinocytes, three of which are characterized in detail. These include:
- EC cells—contact the intestinal lumen, releasing serotonin and motilin to stimulate motility and enhance the production of intestinal juice.
- S cells—respond to the arrival of chyme by producing secretin, which inhibits gastric secretion and triggers the release of pancreatic juice and bile.
- I cells—secrete cholecystokinin (pancreozymin), which stimulates pancreatic and biliary secretion synergistically with S cells.
What secretion is produced by the duodenal (Brunner's) glands?
Duodenal (Brunner's) glands produce a thick, colorless liquid, which is a mucous secretion with a slightly alkaline reaction. The secretion contains:
- Mucus—neutralizes gastric juice, protects the mucosa, and creates an optimal pH for enzymes.
- Biologically active substances—lysozyme and urogastrone (inhibits hydrochloric acid secretion and stimulates cell division).
- Enzymes—enterokinase (converts trypsinogen to trypsin), dipeptidase, amylase, as well as substances with mild proteolytic and lipolytic activity.
The functions of S cells can be remembered by their name: they release Secretin, which Suppresses gastric secretion and Stimulates the release of juices for intestinal digestion.
Class and exam questions
What is the main difference between the wall of the duodenum and those of the stomach and other parts of the intestine?
The main diagnostic feature of the duodenum is the presence of duodenal (Brunner's) glands in the submucosa. The stomach and other parts of the small intestine lack glands in the submucosa.
Where are Paneth cells located and how to identify them on a specimen?
They are located strictly at the base of the intestinal crypts. On a hematoxylin- and eosin-stained specimen, they are easily recognized by the presence of bright pink-red (acidophilic) granules in their apical part.
What is the migration feature of Paneth cells during regeneration?
Unlike most epithelial cells, which originate in the crypts and migrate upward onto the villi for subsequent shedding, Paneth cells differentiate and remain permanently at the base of the crypts.
Where do granules accumulate in endocrine cells of the intestine?
In apudocytes (endocrinocytes), hormone-containing secretory granules accumulate in the basal part of the cell, as their secretion is released into blood capillaries or tissue fluid rather than the intestinal lumen.
What else to study on this topic
- Gradient of goblet cell distribution along the intestine
- Features of the lymphoid apparatus: from solitary follicles to Peyer's patches
- Synergism of secretin and cholecystokinin hormones
- The role of undifferentiated cells in the lower third of the crypts in regeneration
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