44 + 45. Histology - Microanatomy of the Liver, Gallbladder & Pancreas

Liver Basic Concepts

• Liver is part of digestive system, acting as an extramural organ.
• Contains both endocrine and exocrine functions.
• Endocrine function: Synthesis and release of products into blood, like plasma proteins.
• Exocrine function: Production of bile.
• Other functions include detoxification of blood, storage of iron, and metabolism of vitamins A, D, and E.

Liver Structure

• Largest gland in the body.
• Divided into four lobes, though not completely.
• Parenchyma: Organized plates of hepatocytes, main liver cell type, separated by sinusoids, discontinuous capillaries.
• Stroma: Dense irregular connective tissue surrounding the parenchyma.

### Hepatocytes

• Main cell type of the liver.
• Arranged in plates, forming functional units called hepatic lobules.
• Metabolically active, participate in functions related to carbohydrates, proteins, lipids, and hormones.
• Synthesis of plasma proteins, including albumin, globulins, and clotting factors.
• Detoxification of drugs and toxins, using enzymes like cytochrome P450.
• Production and secretion of bile, contains bile acids for fat digestion and bilirubin for removal.

Hepatic Sinusoids

• Discontinuous capillaries located between hepatocyte plates.
• Fenestrated endothelial lining allows for exchange of blood components with hepatocytes.
• Contain Kupffer cells, phagocytic cells that remove particulate matter and old red blood cells from blood.
• Contain space of Disse, a perisinusoidal space between sinusoids and hepatocytes.
• Space of Disse is essential for material exchange between hepatocytes and blood.

Bile Flow

• Bile produced by hepatocytes flows from the bile canaliculi (small channels between hepatocytes) into bile ductules.
• Bile ductules merge into larger intrahepatic bile ducts, lined by columnar epithelium.
• Right and left hepatic ducts collect bile from different liver lobes.
• Common hepatic duct is formed at the hilum (porta) and carries bile to the gallbladder or duodenum.

Lymphatic Flow

• Most lymph originates from the space of Disse.
• Drains from the space of Disse to the periportal space of Mall between the portal connective tissue and hepatocytes.
• Lymph then flows to lymphatic vessels within portal canals and exits the liver at the hilum.

Gallbladder

• Distensible bag connected to the common bile duct via the cystic duct.
• Function: Store and concentrate bile produced by the liver.
• Contains three layers:
  • Mucosa: Innermost layer, made up of simple columnar epithelium with microvilli and a lamina propria.
  • Muscularis: Smooth muscle layer, helps in the expulsion of bile.
  • Serosa: Outermost layer, made up of epithelial and connective tissue.

### Gallbladder Distinguishing Features

• Presence of mucosal folds: Increase surface area to facilitate storage of bile.
• Abundant smooth muscle: Powerful contractions for bile expulsion.
• Absence of villi: Unlike the small intestine.

### Pancreas

• Dual function: Exocrine (production of digestive enzymes) and endocrine (hormonal secretion).
• Exocrine pancreas: Composed of acini, clusters of cells responsible for enzyme production.
• Endocrine pancreas: Contains islets of Langerhans, cell clusters responsible for hormone secretion (like insulin and glucagon).

Exocrine Pancreas Anatomy

• Acini: Clusters of cells that produce and secrete digestive enzymes in the form of zymogen granules.
• Centroacinar Cells: Modified duct cells found in the center of acini, contribute to the pancreatic juice.
• Duct System: Network of ducts that collect and transport the pancreatic juice.
• Intercalated ducts: Smallest ducts, drain the acini, lined by simple cuboidal epithelium, contribute bicarbonate-rich watery secretion.
• Interlobular ducts: Larger ducts located in connective tissue septae, gather fluid from smaller intercalated ducts.
• Main duct: Major duct that carries the pancreatic juice to the duodenum.

Pancreas Endocrine Function

• Islets of Langerhans: Clusters of endocrine cells embedded within the exocrine tissue.
• Alpha cells: Secrete glucagon, raising blood glucose levels.
• Beta cells: Secrete insulin, lowering blood glucose levels.
• Delta cells: Secrete somatostatin, regulating hormone release from other cells.

Pancreatic Enzyme Secretion

• Acinar cells: Secrete proenzymes (inactive digestive enzymes) that are activated by proteases in the duodenum.
• Cholecystokinin (CCK): Hormone secreted from enteroendocrine cells in duodenum, stimulates acinar cell enzyme secretion.
• Acetylcholine: Neurotransmitter released from parasympathetic nerves, stimulates acinar cell secretion.
• Secretin: Hormone secreted from enteroendocrine cells in duodenum, stimulates duct cells to release bicarbonate-rich secretion.

Biliary System and Enterohepatic Circulation

• Biliary system: Network of ducts transporting bile from the liver to the gallbladder and subsequently to the duodenum.
• Enterohepatic circulation: Recycling of bile acids from the intestines back to the liver, enhancing fat digestion.

This quiz covers the basic concepts and structure of the liver, highlighting its vital roles in the digestive system. Learn about the liver's endocrine and exocrine functions, the organization of hepatocytes, and the key metabolic activities of liver cells. Test your knowledge on this essential organ!