Morphological Transition from Stomach to Intestine
Histological slides of the junction between the pyloric part of the stomach and the duodenum clearly demonstrate the change in relief and wall layers. In the stomach, the mucosa forms deep gastric pits, while the muscularis mucosae and muscularis externa consist of three layers. There are no glands in the gastric submucosa (pyloric mucous glands lie in the lamina propria).
Upon transitioning to the duodenum, the picture changes dramatically:
- Broad and short intestinal villi appear, along with invaginations called crypts (crypts of Lieberkühn).
- The number of smooth muscle layers in the lamina propria and muscularis externa decreases to two.
- Duodenal (Brunner's) glands appear in the submucosa, which is the main diagnostic feature of the duodenum.
- Longitudinal folds of the pylorus transition into longitudinal and then circular folds of the intestine.
- Lymphatic follicles are more common 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 never found on the villi. When stained with hematoxylin and eosin, dense acidophilic granules that stain bright pink or red are clearly visible in their apical region.
These cells perform two crucial 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 (APUD cells) are scattered within the epithelium of both crypts (where their density is highest) 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 via paracrine signaling (affecting neighboring cells). There are 6 types of such cells in the intestine, 4 of which are also present in the stomach.
Main types of endocrinocytes:
- EC cells: The most abundant. They respond to chyme entering the intestinal lumen. They produce serotonin and motilin, which stimulate motor function (motility) and enhance intestinal juice secretion.
- S cells: Activated by 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: Release cholecystokinin (pancreozymin). They act synergistically with S cells, powerfully stimulating biliary and pancreatic secretion.
Epithelial Regeneration
Constant physiological renewal of the epithelial layer is critical for gastrointestinal function. In the small intestine, undifferentiated epithelial cells located in the lower half of the crypts act as stem cells (the cambial zone). For comparison, in the stomach, stem cells are located in the neck of the glands, closer to the surface.
- Cells divide at the base of the crypt and begin migrating upward toward the tip of the villus. Upon reaching the tip, older 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, remaining at the base of the crypts.
Differential Diagnosis of Small Intestine Segments
The duodenum has several morphological differences from the jejunum and ileum:
- Submucosa: Brunner's glands are present only in the duodenum.
- Villi: In the duodenum, they are wide and short, whereas in the jejunum and ileum they are thin and tall.
- Folds: Circular folds are frequent and tall in the jejunum, while in the ileum they gradually decrease and disappear.
- Cellular composition: The number of goblet cells in the epithelium increases from the duodenum to the ileum.
- Lymphoid tissue: Solitary follicles are present throughout, but large aggregations—Peyer's patches—form only in the ileum.