Hey All AIPMT Biology Aspirants, next study material is for learning about modifications in layers of Stomach and Small Intestine.

STOMACH: In addition to basic structure of layers of digestive tract, following modifications are present in stomach.

• Epithelial cells of mucosa extend deep down into lamina propria of mucosa, to form columns of secretory cells called Gastric glands. These gastric glands open into the bottom of narrow channels called gastric pits. Secretions from these gastric glands flow into gastric pits to move into the lumen of the stomach.
• Gastric glands contain three types of exocrine gland cells that secrete their products into the stomach lumen: Mucous neck cells, Chief cells or Peptic cells, and Parietal cells or Oxyntic cells.
• Mucous neck cells secrete mucus.
• Chief cells secrete proenzyme pepsinogen and gastric lipase. Also called as Peptic cells.
• Parietal cells produce intrinsic factor (needed for absorption of vitamin B12) and hydrochloric acid (HCl). Also known as Oxyntic cells.
• Secretions of all 3 cells: mucous, parietal, and chief cells form Gastric Juice.

Gastric Pit and Secretory Cells

• Another cell, called as G cell, also present in Gastric glands, secretes the hormone Gastrin into the bloodstream.
• Muscularis of stomach has three layers of smooth muscle (rather than the two found in the esophagus and small and large intestines): an outer longitudinal layer, a middle circular layer, and an inner oblique layer. Oblique layer is limited primarily to the body of the stomach.

Layers of Stomach

SMALL INTESTINE: In addition to basic structure of layers of digestive tract, following modifications are present in small intestine.

• Epithelial cells of mucosa contain many types of cells like Absorptive cells, Goblet cells, Intestinal glands.
• Absorptive cells release enzymes that digest food and contain microvilli that absorb nutrients in small intestine.
• Goblet cells secrete mucus.
• Mucosa of small intestine also contains many deep crevices lined with glandular epithelium. Cells lining these crevices form the Intestinal glands, or Crypts of Lieberkuhn which secrete intestinal juice.
• Further, Intestinal glands contain Paneth cells and three enteroendocrine cells: S cells, CCK cells, and K cells.
• Paneth cells secrete lysozyme (bactericidal enzyme) and are capable of phagocytosis. These cells may have a role in regulating the microbial population in the small intestine.
• S cells secrete secretin hormone, CCK cells secrete cholecystokinin (CCK) hormone, and K cells secrete Glucose-dependent Insulinotropic Peptide (GIP) hormone.
• Lamina propria of mucosa of Ileum contains Lymphatic follicles, or Peyer’s patches in addition to MALT.
• Submucosa of the duodenum contains Duodenal glands, also called as Brunner’s glands which secrete an alkaline mucus that helps neutralize gastric acid in the small intestine.

Small Intestine Layers showing Villi & Intestinal Glands

• Small intestine facilitates the process of digestion and absorption because it includes special structural modifications like circular folds, villi, and microvilli.
• Circular folds are folds of mucosa and submucosa, which enhance absorption by increasing surface area. Food in circular folds move in spiral fashion as it passes through it, rather than moving in a straight line.
• Villi are the tufts of hair or fingerlike projections of the mucosa. Villi vastly increase the surface area of epithelium for absorption and digestion. Each villi has arteriole, venule, blood capillary network, and lacteal (milky), which is a lymphatic capillary.
• Microvilli are projections on the apical (free) membrane of the Absorptive cells. Microvilli form a brush border appearance extending into the lumen of the small intestine. Because the microvilli greatly increase the surface area of the plasma membrane, larger amounts of digested nutrients can diffuse into absorptive cells in a given period.