GIT Lecture PDF

Stomach ,Small and large intestine  stomach The greatly dilated segment of the digestive tract Gross inspection reveals four regions: cardia, fundus, body and pylorus The wall is composed of the 4 major layers Mucosa: lined by a simple columnar epithelium, Underneath...

Stomach ,Small and large intestine  stomach The greatly dilated segment of the digestive tract Gross inspection reveals four regions: cardia, fundus, body and pylorus The wall is composed of the 4 major layers Mucosa: lined by a simple columnar epithelium, Underneath the epithelium is the lamina propria and houses numerous gastric gland Each gland opens into the base of a gastric pit Muscularis mucosae  Mucosa The surface mucous cell Simple columnar epithelial cells with basal nuclei. Line the lumen of the stomach and gastric pits Mucous producing cells a thick, viscous mucus forming hydrophobic layer of gel, that lines the epithelial lining of the stomach and protects it from the highly acidic chyme..  Gastric glands In the fundus and body; The mucosa's lamina propria is filled with branched, tubular gastric glands Each gastric gland has these cells: Mucous neck cells, Parietal cells (oxyntic) Chief (zymogenic) cells. Enteroendocrine/DNES Stem cells  Gastric gland In the neck of the glands are mucous neck cells (MN),, low columnar cells with basophilic, granular cytoplasm and basal rounded nuclei. They secrete mucus. Among the mucous neck cells are stem cells that give rise to all epithelial cells of the glands. They are low columnar cells with basal nuclei, basophilic cytoplasm  Parietal (oxyntic) cells large rounded cells, with large central nuclei surrounded by intensely acidophilic cytoplasm. Parietal cells secrete gastric acid as well as intrinsic factor, a glycoprotein necessary for the absorption of vitamin B12 in the terminal ileum.  Chief (zymogen or peptic) cells The chief cells are small, columnar cells more numerous in the inferior half of the fundic glands and have darkly stained basophilic cytoplasm. Their nuclei are basal and have all the characteristics of protein-synthesizing and - exporting cells. The cytoplasmic granules contain the inactive enzyme for digestion  Enteroendocrine cells Enteroendocrine cells. They release endocrine molecules (e.g. serotonin, gastrin,). They can be distinguished by silver stain. columnar cells It shows dense basal secretory granules.  Stomach Submucosa a richly vascularized connective tissue. Muscularis externa Composed of three layers of smooth muscle: innermost oblique (IO), middle circular (MC) and outer longitudinal (OL). Serosa forms the outermost tunic of the stomach.  SMALL INTESTINE Function: terminal food digestion, nutrient absorption and endocrine secretion of hormones. The small intestine consists of 3 parts: duodenum, jejunum and ileum. They have many characteristics in common. 1- Intestinal Mucosa: Shows finger like projection into the lumen called villi, formed of a core of C.T. covered by epithelium. The core of the villi contains capillary loops, lymphatic channel and few smooth muscle fibers, fibroblasts, macrophages and plasma cells. Crypts of lieberkuhn(intestinal glands) These are simple tubular invagination The mucosa of the small intestine is composed of the usual 3 layers: A simple columnar epithelium, lamina propria and Muscularis mucosa. a) Epithelium: Epithelium covering the villi is simple columnar epithelium formed of 4 types of cells: surface absorptive cells (enterocytes) goblet cells DNES cells M cells (microfold cells). 1- Surface absorptive cells (enterocytes): By L.M.: tall columnar cells with an oval basal nucleus, and apical brush (densely packed microvilli). Function: The microvilli increase the absorptive surface area. The cytoplasm is rich in organelles 2- Goblet cells: Interspersed between absorptive cells. Unicellular glands, least number present in the duodenum. Each cell has a wide distended part and a narrow basal part. The nucleus is located basally. They possess a well developed RER and a well developed Golgi apparatus. Function: produce mucinogen, a protective layer lining the lumen. 3 Diffuse neuroendocrine cells (DNES cells) By L.M.: Cannot be detected by H&E stain. Detected with silver stain, hence the name argentaffin cells. Appear as small cells resting on the basal lamina. Function: They secret cholecystokinin and secretin, which influence pancreas and gallbladder function 4- M cells (microfold cells): The simple columnar epithelium lining of the small intestine is replaced by squamous like M-cells in regions where where lymphoid nodules come in contact with the epithelium. Function: phagocytoses b- Lamina propria: Between the villi are openings of simple tubular glands called crypts of lieberkuhn. In the ileum, the lamina propria contains lymphoid nodules known as Peyer's patches.  Crypts of lieberkuhn: These are simple tubular glands.They are lined by: columnar absorptive cells goblet cells regenerative cells DNES cells and Paneth cells Regenerative (undifferentiated) cells:. These cells are responsible for the production of about 70 billion cells per day that arise within the digestive epithelium. This rapid rate of cell proliferation is required to substitute the continual damage that epithelial cells of the villi and crypts encounter during exposure to stomach acids, food molecules, and intestinal bacteria.  Paneth cells By L.M.: pyramidal shaped with large apical acidophilic secretory granules. Produces Lysozyme and has a role in controlling the bacterial population of the gut  Brunner's gland The submucosa of the duodenum contains Brunner's gland (duodenal glands),which is: The secretory part resembles mucous acini. Function: secrete mucous, alkaline fluid which helps neutralization of the acidic contents entering the duodenum from stomach 4- Serosa: Formed of a C.T. layer with a single layer of squamous epithelium derived from the peritoneal covering. Except for the second and third parts of the duodenum, the entire small intestine is invested by a serosa.  Large intestine Function: absorbs most of the water and ions from the chyme it receives from the small intestine and compacts the chyme into fecal mass to be eliminated. It also secretes mucous to lubricate against the solid fecal mass.  Histology of the colon The colon has no villi but has numerous crypts of lieberkuhn The lining of the crypts contains: - Columnar absorptive cells are the most numerous type numerous goblet cells. (percentage of goblet cells within the lining epithelium is much higher in the large intestine than in the small intestine ) secrete more lubricating mucus. Few DNES cells. Some regenerative cells No Paneth  III- It is unusual in that the outer longitudinal layer is not continuous along the surface but is gathered into 3 narrow bands of muscle fascicles known as teniae coli. The constant tonus maintained by the teniae coli which give the colon its segmented 1. IV-serosa contains appendices epiploicae (fatty tissue is accumulated to form small peritoneal pouches Thank you