12.01.11+-+Gastrointestinal+System+III+-+Pancreas,+Liver,+and+Gallbladder

12.01.11 - Gastrointestinal System III - Pancreas, Liver, and Gallbladder


 * 1. Compare the histologic design of the exocrine pancreas with that of the parotid salivary gland.**

There are a few significant differences visible histologically. The reason that they are being compared in this question is that they are both purely serous glands composed of intercalated ducts and acini. The first difference is the presence of adipocytes in the parotid salivary gland. Another difference is the presence of centroacinar cells penetrating the acinus in the exocrine pancreas. The final important difference is the presence of striated ducts as a mid-step between intercalated ducts and interlobular ducts in parotid glands; exocrine pancreatic glands have no such duct.


 * 2. Describe structure and function of the exocrine pancreas.**

The exocrine pancreas is composed of serous acini and intercalated discs. The serous acini cells secrete the enzymes while the intercalated disc cells secrete the bicarbonate fluid. The discs penetrate into the acini with low cuboidal pale-staining centroacinar epithelium. All secretions flow through pancreatic duct out of the major papilla and into the 1st part of the duodenum.

The function of the exocrine pancreas is to neutralize the acidic chyme with bicarbonate and to secrete enzymes that continue to digest proteins (proteases), fats (lipases), carbs (amylases), and nucleic acids (nucleases).


 * 3. Describe the structure of the pancreatic acinar cell.**

Pancreatic acinar cells are pyramidal-shaped serous cells arranged in berry-shaped secretory units. The nucleus is central and spherical. The apical side has many acidophilic zymogen (enzyme precursor) granules. The baso-lateral side has much RER for protein synthesis.


 * 4. Identify three mechanisms that induce secretion from the exocrine pancreas.**

Two endocrine and one neural inputs modulate the activity of the exocrine pancreas: Secretin: Duodenal APUD cell secretion, activated by presence of chyme, stimulates secretion of bicarbonate-rich fluid by intercalated duct simple cuboidal epithelium. Cholecystokinin: Duodenal APUD cell secretion, activated by presence of chyme, stimulates secretion of enzyme-rich substance from pyramidal acinar cells. Vagus (Parasympathetic) Nerve: Stimulates pancreatic secretion.


 * 5. Compare the effects of secretin and cholecystokinin on pancreatic exocrine products.**

Secretin: Duodenal APUD cell secretion, activated by presence of chyme, stimulates secretion of bicarbonate-rich fluid by intercalated duct simple cuboidal epithelium. Cholecystokinin: Duodenal APUD cell secretion, activated by presence of chyme, stimulates secretion of enzyme-rich substance from pyramidal acinar cells.


 * 6. Describe the histology of the endocrine pancreas. Identify cell types and products released.**

Endocrine pancreas is arranged into islets of Langerhans and dispersed among the predominate exocrine pancreas. The endocrine cells are polygonal in shape with round and central nuclei. The islets are surrounded by a reticular fiber capsule and subserved by fenestrated capillaries.

4 cell types, but 2 important ones: B cells - 60-70% - secrete insulin - stimulated by high blood glucose levels - causes uptake and anabolic activity A cells - 15-20% - secrete glucagon - stimulated by low blood glucose levels - causes release and catabolic activity F, D-1, EC cells - 5% - modulate activity of gut motility, bile secretion, and gastric chief cells (pepsinogen)


 * 7. Describe liver stroma and parenchyma.**

The liver is divided into lobules by a capsule, septa, and vessel tracts. The capsule is specifically called the Capsule of Glisson and is lined by peritoneum except for in the "bare area" against the diaphragm. The septa and tracts separate the liver into lobules and house the vasculature, ducts, nervous, and lymph vessels. The portal triad is a grouping of hepatic artery, portal vein, and bile duct.

The parenchyma is the functionally active portion of the liver composed of hepatocytes, Kuppfer cells, and Ito cells (fat storing cells). Hepatocytes: the principal functional cell of the liver - functionally diverse (protein, lipid, carbohydrate synthesis/modification) - acidophilic polyhedral cells arranged in stacks around central vein. Kuppfer cells: macrophages active in erythrocyte recycling. Ito cells: Storage of Vitamin A - active in liver regeneration.


 * 8. Summarize blood flow to, through and from the liver.**

Blood flows into the liver from the hepatic portal vein (80%) and the hepatic artery (20%). It disperses through the liver into portal triads that contain both a hepatic artery and a hepatic portal venule. The artery and venule branch into hepatic sinusoids and arteriole branches that merge into the central vein of their respective central vein. Blood then flows through the central vein and out into the hepatic veins and then into the inferior vena cava.


 * 9. Describe the structure and components of the classic liver lobule.**

The lobule is a polygonal unit with 3-6 portal areas surrounding a central vein. It is surrounded by a poorly defined connective tissue capsule. Each portal area has a portal tract associated with it.


 * 10. Identify ducts of the biliary tree.**

The biliary tree flows in the reverse direction of the blood flow. It originates in canaliculi formed by tight junctions between hepatocytes. The canaliculi merge and flow into branches called bile ductules that flow into bile ducts. Bile ducts merge to form the hepatic duct, which comes together with the cystic duct from the gallbladder to form the common bile duct that drains into the duodenum. They are simple cuboidal epithelium.


 * 11. Describe the structure and functions of hepatocyte.**

Hepatocytes: the principal functional cell of the liver - functionally diverse (protein, lipid, carbohydrate synthesis/modification) - acidophilic polyhedral cells arranged in stacks around central vein.

They have 1-2 nuclei, 2000 mitochondria, 50 golgi, and much of everything else: RER, SER, peroxisomes, lipid droplets, and glycogen.


 * 12. Describe the histologic structure and function of the gallbladder.**

The gallbladder is a storage bag for bile produced by the liver; it also absorbs water to condense the bile within the bladder. The wall of the gallbladder:

simple colunar epithelium lamina propria thin muscularis (disorganized fibers) adventitia (on liver side) or serosa (on external side)