1. Mucosa - as in most other parts of the GI tract consists of 3 layers.

2. Submucosa - loose irregular c.t. containing blood and lymph vessels, a submucosal nerve plexus (Meissner's) and submucosal mucous glands (esophageal). Ganglion cells of the parasympathetic and post ganglion fibers of the sympathetic systems compose the nerve plexus.

3. Muscularis externa - follows the general plan of organization: an inner circular and outer longitudinal layer of muscle with the autonomic myenteric nerve plexus (Auerbach's) in between the layers. The upper 1/4 of the esophagus is skeletal, middle 1/4 is mixed, and the lower 1/2 is smooth muscle.

4. Adventitia - a connective sheath. Where it is covered by mesothelium it is called a serosa; otherwise, it is called a fibrosa. Little, if any, serosa is associated with the esophagus.

1. Region of fundus and body - gastric glands in this region are called fundic (but more appropriately peptic) glands. They are relatively straight tubular glands slightly coiled at their blind end. They empty into the pits.

The glands consist of:

2. Pyloric region - the pyloric (mucous) glands occupy the outer half of the mucosa; pits occupy the inner half. The mucous secretory glands are coiled tubes. The cells are similar to the mucous neck cells.

3. Cardiac region (or junction) - transition from esophagus to stomach. A very narrow region in the stomach contains mucosal, mucous-secreting cardiac glands.

4. Pyloric junction - transition from pyloric stomach to duodenum with the appearance of submucosal, pure mucous, Brunner's glands, and intestinal villi.

1. Epithelium - simple, tall, columnar epithelium consisting of absorptive cells interspersed with goblet cells on the villi and, in the crypts, proliferating and differentiating cells. Absorptive and goblet cells develop in the crypts and migrate to the tip of the villus where they are shed - time: about 5 days.

2. Lamina propria - especially rich in diffuse lymphoid tissue with frequent solitary nodules. Aggregates of nodules (Peyer's patches) occur in the terminal ileum. These extend into the submucosa.

3. Muscularis mucosa - follows general plan

1. Brunner's glands - pure mucous submucosal glands are limited to the duodenum. They are most concentrated near the stomach and secrete an alkaline mucus. They also produce urogastrone, which inhibits gastric acid secretion and stimulates epithelial proliferation.

1. Histology - a pure serous gland. It secretes enzymes which digest protein, carbohydrate, fat and nucleic acids. Each acinus partly surrounds the terminal portion of an intralobular duct. The duct cells thus surrounded by acinar cells are called centroacinar cells. Intralobular ducts are inconspicuous and open into the conspicuous interlobular ducts. These are conspicuous because they are surrounded by a dense connective tissue sheath. This sheath is the primary support for the pancreas, since there is very little intralobular connective tissue and the capsule is very thin.

2. Control - stimulation by the vagus and cholecystokinin - when acid chyme enters the intestine, cholecystokinin is released which, with increased vagal activity, stimulates the release of an enzyme rich secretion. Secretin, another intestinal hormone, stimulates the centro-acinar cells and intralobular ducts to release a secretion poor in enzymes and rich in bicarbonate.

1. Alpha cells produce glucagon, which elevates blood glucose.

2. Beta cells produce insulin, which lowers blood glucose. Pro-insulin is formed in the RER, which is split in the Golgi region to insulin and packaged for secretion. Secretion of insulin is stimulated by elevated blood glucose and modified by parasympathetic and sympathetic nerves.

3. Delta cells produce somatostatin. The role of somatostatin may be to regulate other secretions.

1. Capsule - a fibrous connective tissue layer surrounding the liver and covered by mesothelium except next to the diaphragm. It is thickened in the region of the porta where the vessels and nerves enter and leave the organ. This connective tissue forms an extensive branching system which surrounds the vessels and nerves it accompanies. Together these structures form a tree-like support of the liver. Its branches are called portal canals, but when seen in cross-section, they are named portal areas. Little c.t. is associated with the hepatic veins and especially their terminal branches, the central veins. Two interlacing tree-like patterns are formed by these vessels; the hepatic veins emptying into the inferior vena cava, and the portal vein and its branches, which arise from the porta.

2. Portal canals

1. Hepatocytes - large polygonal shaped, epithelial cells. They have fine chromatin in their nuclei and a finely granular cytoplasm which may show vacuolation due to glycogen and fat. Ultrastructurally they contain scattered mitochondria with few cristae, RER and SER, which respond to functional demands, lysosomes and peroxisomes. Bile canaliculi are formed by the plasma membrane of adjacent hepatocytes and are surrounded by actin filaments. The actin can alter the size and shape of the canaliculi. J. Histo & Cytochem: 41:353, 1993. Tight junctions between these membranes prevent the bile from leaking out of the canaliculi into the perisinusoidal space of Disse' and thus into the blood. Irregular microvilli extend from the hepatocyte surface into the space of Disse' and are bathed by plasma..

2. Organization - the hepatocytes form irregular, branching plates or cords which are usually only a single cell thick. Sinusoids lined by an endothelium with an incomplete basal lamina and having fenestrations of various sizes surround these plates and cords. The endothelial cells are separated from the hepatocytes by the space of Disse' in which a few fibrocytes, lipocytes and reticular fibers may be found. The lipocytes may be the source of fibroblasts especially in cirrhosis. Inside the sinusoids stellate shaped macrophages (Kupffer cells) abound and are a significant component of the mononuclear phagocytic system. The cords and plates are arranged radially around a central vein toward which blood flows in the sinusoids. There are three ways of defining the fundamental unit of liver structure.

3. Functional considerations

1. Function - primarily concentrates bile by the active transport of sodium ions across the lateral and basal cell membranes of the epithelial cells, anions and water follows the sodium.

2. Epithelium - a very characteristic simple, tall columnar epithelium with many irregular microvilli on the apex. Junctional complex and scattered desmosomes anchor the cells together, but space between cells is widened during active transport.

3. Lamina propria - when the muscularis contracts, the mucosa is thrown into complex folds so the epithelially lined pockets become surrounded by lamina propria. In the neck region, mucous-secreting glands may be found.