Histology of Digestive system Lecture II PDF

Histology of Digestive system Stomach Prof. Dr. Soad A. Treesh. Spring 2024  Stomach A mixed exocrine– endocrine organ.  It is a dilated segment of the digestive tract.  Gross inspection reveals four regions: cardia, fundus, body, and pylorus. ...

Histology of Digestive system Stomach Prof. Dr. Soad A. Treesh. Spring 2024  Stomach A mixed exocrine– endocrine organ.  It is a dilated segment of the digestive tract.  Gross inspection reveals four regions: cardia, fundus, body, and pylorus.  Because the fundus and body are identical in microscopic structure, only three histological regions are recognized.  The mucosa and submucosa of the undistended stomach lie in longitudinally directed folds known as rugae.  When the stomach is filled with food, these folds flatten out.  Cardia: a narrow region at the gastroesophageal junction, 2 to 3 cm wide  Fundus: a dome-shaped region to the left of the esophagus, frequently filled with gas  Body (corpus): the largest portion, responsible for the formation of chyme  Pylorus (pyloric antrum): a funnel-shaped, constricted portion equipped with a thick pyloric sphincter that controls the intermittent release of chyme into the duodenum  Functions of stomach  Main functions are to continue the digestion of carbohydrates initiated in the mouth, add an acidic fluid to the ingested food, transform it by muscular activity into a viscous mass (chyme), and promote the initial digestion of proteins with the enzyme pepsin. It also produces a gastric lipase that digests triglycerides with the help of lingual lipase.  Gastric histology  The epithelial lining of the stomach secretes visible mucus that adheres to and protects the stomach lining.  The epithelial lining of the stomach invaginates into the mucosa, forming gastric pits (foveolae), which are shallowest in the cardiac region and deepest in the pyloric region.  Gastric pits increase the surface area of the gastric lining.  5-7 gastric glands of the lamina propria empty into the bottom of each gastric pit.  Cardia  A narrow circular band, 1.5–3 cm in width.  Its mucosa contains simple or branched tubular cardiac glands.  Most of the secretory cells produce mucus and lysozyme (an enzyme that attacks bacterial walls), but a few parietal cells secreting H+ and Cl–.  These glands are similar in structure to the cardiac glands of the terminal portion of the esophagus.  Stomach, cardia *The cardiac mucosa contains mucous-secreting cells (simple columnar epithelium) (Surface mucous cells). *Short gastric, *Gastric glands. *The lamina propria and muscularis mucosa, 9  Fundus & Body  The lamina propria of the fundus and body is filled with branched, tubular gastric (fundic) glands, 3-7 of which open into the bottom of each gastric pit.  Each gastric gland has three distinct regions: the isthmus, neck, and base.  The isthmus, close to the gastric pit, contains differentiating mucous cells, undifferentiated stem cells, and oxyntic (parietal) cells.  The neck of the glands consists of stem, mucous neck (different from the mucous cells in the isthmus), and parietal cells.  The base of the glands primarily contains parietal and chief (zymogenic) cells. Enteroendocrine cells are dispersed in the neck and base of the glands. Stomach  Stem Cells  Found in the isthmus and neck regions but few in number, stem cells are low columnar cells with oval nuclei near the bases of the cells.  These cells have a high rate of mitosis; some of them move upward to replace the pit and surface mucous cells, which have a turnover time of 4–7 days.  Other daughter cells migrate more deeply into the glands and differentiate into mucous neck cells and parietal, chief, and enteroendocrine cells.  These cells are replaced much more slowly than surface mucous cells.  Mucous Neck Cells  Mucous neck cells are present in clusters or as single cells between parietal cells in the necks of gastric glands. Their mucus secretion is quite different from that of the surface epithelial mucous cells. They are irregular in shape, with the nucleus at the base of the cell and the secretory granules near the apical surface.  Oxyntic (Parietal) Cells  Present mainly in the upper half of gastric glands; they are scarce in the base.  They are rounded or pyramidal cells, with one centrally placed spherical nucleus and intensely eosinophilic cytoplasm.  Abundance of mitochondria (eosinophilic) and a deep, circular invagination of the apical plasma membrane, forming the intracellular canaliculus.  In the resting cell, a number of tubulovesicular structures can be seen in the apical region just below the plasmalemma.  At this stage, the cell has few microvilli. When stimulated to produce H+ and Cl–, tubulovesicles fuse with the cell membrane to form the canaliculus and more microvilli, thus providing a generous increase in the surface of the cell membrane.  Chief (Zymogenic) Cells  Predominate in the lower region of the tubular glands and have all the characteristics of protein- synthesizing and -exporting cells.  Their basophilia is due to the abundant rough endoplasmic reticulum. The granules in their cytoplasm contain the inactive enzyme pepsinogen.  The precursor pepsinogen is rapidly converted into the highly active proteolytic enzyme pepsin after being released into the acid environment of the stomach.  Chief cells also produce the enzyme lipase. Enteroendocrine Cells  Found in the neck and bases of gastric glands.  In the fundus of the stomach, 5-hydroxytryptamine (serotonin) is one of the principal secretory products.  Other products of enteroendocrine cells in the gastrointestinal tract.  Pylorus  Has deep gastric pits into which the branched, tubular pyloric glands open. Pyloric glands have longer pits and shorter coiled secretory portions.These glands secrete mucus as well as appreciable amounts of the enzyme lysozyme.  Gastrin (G) cells (which release gastrin) are enteroendocrine cells intercalated among the mucous cells of pyloric glands.  Other enteroendocrine cells (D cells) secrete somatostatin, which inhibits the release of some other hormones, including gastrin.  Secretion of somatostatin is stimulated by HCl, counterbalancing the acid secretion. Other Layers of the Stomach  The submucosa is composed of dense connective tissue containing blood and lymph vessels; it is infiltrated by lymphoid cells, macrophages, and mast cells.  The muscularis is composed of smooth muscle fibers oriented in three main directions. The external layer is longitudinal, the middle layer is circular, and the internal layer is oblique. At the pylorus, the middle layer is greatly thickened to form the pyloric sphincter. The stomach is covered by a thin serosa.  Pyloric-duodenal junction  The Pyloric-duodenal junction or gastroduodenal junction is the boundary between the pyloric stomach and the duodenum of the small intestine. The pyloric sphincter controls the passage of chyme from the stomach into the duodenum.  The stomach and the duodenum are composed of the four layers characteristic of the gastrointestinal tract.  Pyloric glands are located in the mucosa whereas the duodenal glands are located in the submucosa.  Mucosa (or mucous membrane)  Epithelium - structure abruptly changes:  Pyloric Stomach - long pits and short, coiled glands.  Duodenum - broad projections (or villi) and crypts.  Muscularis Mucosae - continuous between the pyloric stomach and the duodenum.  Submucosa - contains Brunner's glands in the duodenum.  Muscularis Externa - expands in the stomach to form the pyloric sphincter. 27  End of Part II Good luck