Lecture 13: Histology of Liver, Gallbladder, and Pancreas

Liver, Gallbladder, and Pancreas Overview

• This histology lecture covers the liver, gallbladder, and pancreas
• Covered in Histology, chapter 18

Learning Objectives

• Describe the functions of the liver, gallbladder, and pancreas
• Identify features of the liver, liver lobules, and blood supply in histological sections
• Identify ultrastructural features and cells of the liver in electron microscopic images
• Describe the main cell types function in the liver
• Describe the flow/path of arterial and venous blood through the liver
• Describe the flow of bile from the hepatocyte to the common bile duct
• Identify features of the gallbladder in histological sections
• Identify endocrine and exocrine components in histological sections of the pancreas
• List functions of main exocrine and endocrine cell types in the pancreas
• Compare and contrast the endocrine and exocrine cells of the pancreas
• Differentiate histologically between the liver, gallbladder, pancreas, and salivary glands

Liver Physiology

• Produces key circulating plasma proteins: albumin, lipoproteins, prothrombin, fibrinogen
• Stores and converts lipid-soluble vitamins like A, D, E, and K using hepatic stellate cells
• Functions in iron storage, metabolism, and homeostasis
• Degrades toxins and drugs, converting them into soluble forms for kidney elimination via oxidation and conjugation
• Removes damaged blood cells from circulation using Kupffer cells
• Involved in carbohydrate, lipid, and protein metabolism
• Absorbs glucose from GI tract and stores it as glycogen, which is broken down via glycogenolysis to release glucose
• Bile is produced
• Consumes fatty acids from plasma

Portal Triad Structures

• Consists of a hepatic arteriole, portal venule, and bile duct within a connective tissue sheath
• Hepatic arterioles supply oxygenated blood to the liver
• Portal venules carry nutrient-rich blood from the digestive system to the liver for processing
• Bile ducts transport bile produced by hepatocytes away from the liver

Classic Hepatic Lobule

• Drains blood from the portal vein and the hepatic artery to the hepatic or central vein

Liver Cells: Hepatocytes

• Polyhedral in shape
• Comprise approximately 80% of the liver's cell population
• Each hepatocyte contains a large nucleus at its center

Hepatocyte Functions

• Rough endoplasmic reticulum (rER) synthesizes plasma proteins for release into the perisinusoidal space
• Toxic compounds, bilirubin, and bile acids are taken up from the perisinusoidal space, processed by smooth endoplasmic reticulum (sER) enzymes, and secreted into bile canaliculi
• Glucose is taken up from the perisinusoidal space, stored in glycogen granules, and reversed when glucose is needed

Hepatic Sinusoids

• Have a thin, discontinuous epithelium
• Contain stellate sinusoidal macrophages, or Kupffer cells, which:
  • Are mononuclear phagocytic macrophages
  • Are part of the sinusoid lining
• Hepatic stellate cells known as Ito cells, store vitamin A
  • Located in the perisinusoidal space (not the lumen)
  • Can differentiate into myofibroblasts under pathological conditions

Perisinusoidal Space (of Disse)

• The space between hepatocytes and the basal surface of endothelial cells
• Facilitates the exchange of materials between hepatocytes and plasma
• Characterized by gaps in endothelial cells facilitate exchange and microvilli

Bile Canaliculi

• Channels located between hepatocytes
• Separated by narrow spaces known as hepatic sinusoids

Lymph and Biliary Pathways

• Bile canaliculi drain into the canal of Hering, which then drains into the intrahepatic bile ductule
• Consists of hepatocytes and cholangiocytes
• Periportal space (of Mall) facilitates lymph travel in the same direction as bile
• Drains plasma from the perisinusoidal space

Intrahepatic Ductile

• Cholangiocytes line this structure
• Basal lamina present
• Asterisks denote the periportal space of Mall

Cholangiocytes Function and structure

• Epithelial cells that line the bile canaliculi
• Have few organelles
• Tight junctions and a complete basal lamina
• Have microvilli projecting into the lumen
• Long cilia that detect bile flow direction
• Can alter cholangiocyte secretion

Biliary Tree Components

• Bile canaliculi: Located between hepatocytes
• Canals of Hering: Lined by hepatocytes and cholangiocytes
• Intrahepatic bile ductules: Lined by cholangiocytes
• Interlobular bile ducts: Lined by cholangiocytes
• Left and right hepatic ducts: Columnar epithelial cells
• Common hepatic duct and the Cystic duct

Bile Composition and Function

• Involved in fat absorption and the excretion of cholesterol, bilirubin, iron, and copper
• 90% of bile is absorbed by the gut and transported back to the liver
• 10% of bile is synthesized de novo (from scratch)

Gallbladder Function and Structure

• Concentrates and stores bile
• Lacks muscularis mucosa and submucosa
• Lamina propria characterized by mucin-secreting glands
• Simple columnar epithelium
• Only has muscularis externa with randomly arranged muscle fibers, which enable contraction to expel bile

Gallbladder Bile Concentration

• Junctional complexes create a barrier between the lumen and the intercellular compartment
• Simple columnar epithelium
• Epithelial cells transport Na⁺, Cl⁻, and HCO3⁻ into the intercellular compartment
• This transport creates an osmotic gradient
• Arrows indicate distension of the intercellular space from fluid transport

Pancreas

• Has endocrine and exocrine functions

Pancreas: Endocrine Function

• Secretes insulin and glucagon into the blood
• Located in large groups of cells called Islets of Langerhans, dispersed in the pancreas

Pancreas: Exocrine Function

• Digestive enzymes secreted into the duodenum
• Located throughout the pancreas
• Serous acinar gland

Pancreatic Acinus Function and Structure

• Acinar cells secrete enzymes, stored as zymogen granules, draining into an intercalated duct
• Enzymes include:
  • Proteolytic endo- and exopeptidases to digest proteins
  • Amylolytic enzymes to digest carbohydrates
  • Lipases to digest lipids
  • Nucleolytic enzymes to digest nucleic acids
• Enzymes only become active upon entering the lumen of the small intestine

Pancreas Cell Types: Acinar Cells

• N- nuclei
• Z- zymogen granules
• rER- rough ER
• M- mitochondria
• G- golgi profiles
• JC- junction complex
• L- lumen

Intercalated Duct in the Pancreas

• Contains centroacinar cells
• Contains intercalated duct cells
• Adds bicarbonate and water to exocrine secretions

Islets of Langerhans

• Endocrine component of the pancreas
• ~1-2% of the volume of the pancreas
• B (beta) cells (~60-70%)- secrete insulin
• A (alpha) cells (~15-20%)- secrete glucagon
• D (delta) cells (~5-10%)- secrete somatostatin