Digestive System II Esophagus Stomach

Esophagus 1. The esophagus is lined by a stratified squamous nonkeratinized epithelium. 2. The lamina propria contains mucus-secreting esophageal cardiac glands, and the submucosa contains mucus-secreting esophageal glands proper. 3. The muscularis mucosae varies in thickness and is composed of a single longitudinal layer of smooth muscle. 4. The upper third of the muscularis externa contains skeletal muscle; the middle third contains a combination of smooth and skeletal muscle; and the lower third contains smooth muscle. 5. The esophagus conveys a bolus of food from the pharynx into the stomach by peristaltic activity of the muscularis externa. Two physiologic sphincters (the pharyngoesophageal and the gastroesophageal) in the muscularis externa ensure that the bolus is transported in one direction only, toward the stomach.

Photomicrograph of a section of the upper region of the esophagus Photomicrograph of a section of the upper region of the esophagus. Mucous esophageal glands are in the submucosa; striated skeletal muscle is in the muscularis. PAS and PT stain. Low magnification.

Stomach Function General structure The stomach acidifies and converts the bolus into a thick, viscous fluid known as chyme. It also produces digestive enzymes and hormones. General structure a. The stomach exhibits longitudinal folds of the mucosa and submucosa (called rugae), which disappear in the distended stomach. b. It has many gastric pits (foveolae), which are shallowest in the cardia and deepest in the pylorus.

Regions of the stomach and their histological structure.

Gastric mucosa (a) The simple columnar epithelium of the gastric mucosa is composed of mucinogen-producing surface lining cells (not goblet cells). (b) The lamina propria is a loose connective tissue housing smooth muscle cells, lymphocytes, plasma cells, mast cells, and fibroblasts. It contains gastric glands. (c) The muscularis mucosae is composed of a poorly defined inner circular layer, an outer longitudinal layer, and, occasionally, an outermost circular layer of smooth muscle.

Gastric submucosa: (a) Is composed of dense irregular collagenous connective tissue (b) Contains fibroblasts, mast cells, and lymphoid elements embedded in the connective tissue (c) Houses Meissner (submucosal) plexus (d) Possesses arterial and venous plexuses that supply and drain the vessels of the mucosa, respectively

Gastric muscularis externa: (a) Is composed of three layers of smooth muscle: an incomplete inner oblique layer; a thick middle circular layer, which forms the pyloric sphincter; and an outer longitudinal layer. Auerbach myenteric plexus is located between the middle circular and outer longitudinal smooth muscle layers. (b) Is responsible for mixing of gastric contents and emptying of the stomach (c) Is affected by various characteristics of the chyme (e.g., lipid content, viscosity, osmolality, caloric density, and pH), which influence the emptying rate of the stomach and the rate of muscle contraction A serosa covers the external surface of the stomach.

Photomicrograph of a section of the gastric glands in the fundus of the stomach. Note the superficial mucus-secreting epithelium. Parietal cells (light-stained) predominate in the mid and upper regions of the glands; chief (zymogenic) cells (dark-stained) predominate in the lower region of the gland. MM, muscularis mucosae. PT stain. Low magnification.

Gastric glands are simple branched tubular glands located in the lamina propria of the cardia, fundus, body, and pylorus. Each gland consists of an isthmus, which connects the gland to the base of a gastric pit, a neck, and a base.

Cells of the fundic and body glands (1) Parietal (oxyntic) cells (a) These are pyramid-shaped cells concentrated in the upper half ofthe gland. (b) They secrete hydrochloric acid (HCl) and gastric intrinsic factor. The latter is necessary for absorption of vitamin B 12 in the ileum. (c) They possess a unique intracellular tubulovesicular system, many mitochondria, and secretory intracellular canaliculi (deep invaginations of the apical plasma membrane) lined by microvilli. (d) When stimulated to secrete HCI, the number and length of microvilli increase and the complexity ofthe tubulovesicular system decreases (suggesting that tubulovesicle membranes are incorporated into the intracellular canaliculi, thus lengthening the microvilli).

Photomicrograph of a mucus-secreting surface epithelium (A) and mucous neck cells intercalated between oxyntic (parietal) cells located in the mid portion of the gastric gland (B). Abundant capillaries can be seen. PT stain. Medium magnification.

Composite diagram of a parietal cell, showing the ultrastructural differences between a resting cell (left) and an active cell (right). Note that the tubulovesicles (TV) in the cytoplasm of the resting cell fuse to form microvilli (MV) that fill up the intracellular canaliculi (IC). G, Golgi complex; M, mitochondria. (Based on the work of Ito S, Schofield GC. J Cell Biol 1974;63:364.)

(2) Chief (zymogenic) cells: (a) Are pyramid-shaped cells located in the lower half of the gland (b) Secrete pepsinogen (a precursor of the enzyme pepsin) and the precursors of lipase (c) Display an abundance of basally located rough endoplasmic reticulum (RER), a supranuclear Golgi complex, and many apical zymogen (secretory) granules (3) Mucous neck cells: (a) Are located in the neck of the gland (and may be able to divide) (b) Possess short microvilli, apical mucous granules, a prominent Golgi complex, numerous mitochondria, and some basally located RER

Photomicrograph of the basal portion of the gastric gland in the fundus. This section shows parietal cells rich in mitochondria and their characteristic intracellular canaliculi (arrowheads). Chief cells show red secretory granules in their cytoplasm. PT stain.

Photomicrograph of a mucus-secreting surface epithelium (A) and mucous neck cells intercalated between oxyntic (parietal) cells located in the mid portion of the gastric gland (B). Abundant capillaries can be seen. PT stain. Medium magnification.

(4) Diffuse neuroendocrine cells (DNES cells): (a) Are also referred to as enteroendocrine cells or as APUD cells (amine precursor uptake and decarboxylation cells) (b) Include more than a dozen different types of cells that house many small hormone-containing granules, usually concentrated in the basal cytoplasm. A given enteroendocrine cell is believed to secrete only one hormone (c) Possess an abundance of mitochondria and RER and a mod- erately well-developed Golgi complex (5) Regenerative cells are located primarily in the neck and isthmus; they replace all the epithelial cells of the gland, gastric pit, and luminal surface.

Cardiac and pyloric glands are different from fundic glands in that they are coiled tubular mucus-secreting glands and lack chief cells. Gastric juice contains water, HCI, mucus, pepsin, lipase, rennin, and electrolytes. It is very acidic (pH 2.0) and facilitates the activation of pepsinogen to pepsin, which catalyzes the partial hydrolysis of proteins.

Regulation of gastric secretion It is effected by neural activity (vagus nerve) and by several hormones. a. Gastrin, released by enteroendocrine cells in the gastric and duodenal mucosa, together with histamine and acetylcholine, stimulates HCl secretion. b. Somatostatin, produced by enteroendocrine cells of the pylorus and duodenum, inhibits the release of gastrin and thus indirectly inhibits HCl secretion.