Classic liver lobule in digestive system This model is based on the direction of blood flow. In sections, liver substructure exhibits a pattern of interlocking hexagons; each of these is a classic lobule. Whereas lobules in pigs are defined by a sheath of connective tissue, there is less connective tissue in humans and the lobule boundaries are indistinct. The boundaries of human lobules can be estimated, however, by noting the positions of the portal triads at the lobule periphery, the central vein at its center, and the alternating hepatocyte plates and sinusoids that lie between them. a. Portal triad. One triad occupies a potential space (portal space) at each of the 6 corners of the lobule. Each triad contains 3 main elements surrounded by connective tissue: a portal venule (a branch of the portal vein), a hepatic arteriole (a branch of the hepatic artery), and a bile ductule (a tributary of the larger bile ducts). A lymphatic vessel may also be seen. b. Central vein. A single vein marks the center of each lobule. This vessel is easily distinguished from those in the portal triad by its larger opening and lack of a connective tissue investment. c. Hepatocyte plates and hepatic sinusoids. Many such plates radiate from the central vein toward the lobule periphery (like the spokes of a wheel). The plates are separated by hepatic sinusoids, which receive blood from the vessels in the triads, converging on the lobule center to empty directly into the central vein.
Liver Lobules
LIVER
A. General Structure: The liver is the body's largest gland. It is partly covered by a thin capsule (Glisson's capsule) and has a sparse, delicate, reticular connective tissue stroma accompanying the blood vessels as they penetrate the parenchyma. Its predominant cell type is the hepatocyte. These cells are arranged in one- or 2-cell-thick plates that are separated by the hepatic sinusoids. The liver has a dual blood supply, the portal vein and the hepatic artery; it also has 3 drainage systems, the hepatic veins, lymphatic vessels, and bile ducts.
B. General Functions: The liver has several important functions, most of which are carried out by hepatocytes. Its main role in digestion involves the enzymatic processing (metabolism) of nutrients absorbed by the intestines to provide the body with the chemical building blocks and fuel needed to support life. Some hepatocyte enzymes aid in detoxification by modifying potentially dangerous chemicals and drugs and rendering them harmless. Hepatocytes synthesize many important proteins; albumin, prothrombin, fibrinogen, lipoproteins) and secrete them into the blood, thus acting as an endocrine gland. They also synthesize bile from the wastes of erythrocyte destruction and secrete it into the biliary tract, acting as an exocrine gland. The liver also serves as a storage site for glucose, fats, and vitamin A
nC. Blood Supply:
n1. Hepatic portal vein. 2. Hepatic artery. 3. Hepatic sinusoids. 4. Central veins. 5. Hepatic veins.
nD. Cell Types:
n1. Hepatocytes. 2. Kupffer's cells. Monocyte-derived members of the mononuclear phagocyte system. 3. Fat-storing cells.
Cell Types : Pancreatic acinar cells 2. Endocrine cells.
Digestive System
1. Pancreatic acinar cells. Each acinus consists of several pyramid-shaped, enzyme secreting cells whose apices border on a small lumen and whose bases abut a basal lamina. Acinar cells synthesize a wide variety of enzymes that can hydrolyze proteins (proteases, such as trypsin, chymotrypsin, and elastase), lipids (lipases, such as triacylglycerol lipase and phospholipase A,), carbohydrates (amylase), and nucleic acids (ribonuclease and deoxy ribonuclease). Enzymes are packaged and stored in the acidophilic apical region as membrane-bound zymogen granules. Here they await exocytosis in response to stimulation by cholecystokinin, which is produced by enteroendocrine cells in the small intestine, or parasympathetic stimulation via the vagus nerve. The enzymes in the granules are zymogens, which are inactive before the release. One such zymogen, trypsinogen, is enzymatically converted to the active protease trypsin in the small intestine by enterokinase, an enzyme that is secreted by enterocytes.
2. Endocrine cells.
PANCREAS
General Structure and Function of Pancreas : The pancreas is a serous, compound acinar gland that resembles the parotid gland in its microscopic appearance. important member of digestive system.
It differs in that it lacks striated ducts and contains islets of Langerhans.
The lobules of the pancreas contain serous adenomeres that secrete a variety of digestive enzymes into a branched duct system that empties into the duodenum.