Microscopic structure of liver histology 5m answer

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Microscopic Structure of the Liver

Yamada's Textbook of Gastroenterology, 7th ed.; Sleisenger & Fordtran's GI and Liver Disease

Normal liver structure diagram showing portal tracts, sinusoids, space of Disse, bile canaliculi, hepatic veins, and limiting plate

1. Hepatic Lobule - Structural Models

Two models describe the liver's microscopic organization:

Classic (Hexagonal) Lobule

A hexagonal unit centered on a central vein (terminal hepatic venule), surrounded by six peripheral portal tracts at each corner.
Hepatocyte plates radiate from the central vein toward the portal triads.

Rappaport Acinus (functional model)

Based on blood supply; centers on the portal tract.
Divided into three zones based on oxygen gradient:
- Zone 1 (periportal) - best oxygenated; first to receive portal blood; resistant to ischemia; first affected in viral hepatitis.
- Zone 2 (mid-zonal) - intermediate.
- Zone 3 (centrilobular/perivenular) - least oxygenated; most susceptible to ischemic/toxic injury; highest cytochrome P450 activity; first to show centrilobular necrosis in CCl4 toxicity or right heart failure.

2. Portal Tract (Portal Triad)

Each portal tract contains three structures - the "portal triad":

Portal venule - largest of the three; ~3x the diameter of the arteriole or bile duct
Hepatic arteriole - approximately the same caliber as the bile duct (useful diagnostically)
Interlobular bile duct - lined by a single layer of cuboidal cells; contains cytokeratins 7 and 19

The portal tract is surrounded by a limiting plate - the interface between portal connective tissue and the hepatic parenchyma. Inflammatory cells crossing this plate produce "interface hepatitis."

3. Parenchyma - Hepatocytes

Hepatocytes make up ~80% of liver cells. Key features:

Arranged in cords (trabeculae) 1-2 cells thick in adults, each flanked by sinusoids on both sides
Nucleus: centrally placed, round, ~10 µm, with clumped chromatin and prominent nucleoli; binucleate forms seen in zone 3 of elderly
Cytoplasm: contains abundant organelles:
- ~2200 mitochondria per hepatocyte (larger in zone 3, more numerous in zone 1)
- Rough ER - protein synthesis (albumin, coagulation factors)
- Smooth ER - fatty acid metabolism, drug detoxification via cytochrome P450 (highest in zone 3)
- Golgi apparatus - glycoprotein and lipoprotein processing
Bile canaliculi: grooves (hemicanaliculi) between adjacent hepatocytes fuse to form canaliculi; surrounded by actin microfilaments enabling peristaltic bile propulsion; visible on scanning EM

4. Sinusoids and Space of Disse

Sinusoids are wide, irregular vascular channels that carry mixed portal and arterial blood from the portal tracts toward the central vein.
Lined by discontinuous, fenestrated endothelial cells (no basement membrane), allowing direct plasma-hepatocyte contact.
The Space of Disse lies between the sinusoidal endothelium and the hepatocyte surface. It contains:
- Plasma (not blood cells)
- Reticulin fibers (type III collagen) maintaining the lobular framework
- Hepatic stellate cells (Ito cells/fat-storing cells) in a resting state

5. Sinusoidal Cell Types

Cell Type	Location	Function
Kupffer cells	Lining sinusoids	Resident macrophages; phagocytosis of debris, bacteria, endotoxins
Hepatic stellate cells (Ito cells)	Space of Disse	Store vitamin A (lipid droplets) in resting state; activated → myofibroblasts → collagen synthesis → fibrosis
Pit cells	Sinusoid lumen	Large granular lymphocytes; liver-specific NK cells
Sinusoidal endothelial cells	Sinusoid lining	Fenestrated, no basal lamina; facilitate exchange

6. Biliary Network

Bile flows counter to blood flow, from hepatocytes outward toward portal tracts:

Bile canaliculi - between hepatocyte surfaces (not a separate structure)
Canals of Hering - near the limiting plate; lined partly by hepatocytes and partly by biliary epithelium; serve as the hepatic stem cell niche
Terminal bile ductules (cholangioles)
Interlobular bile ducts in portal tracts (15-20 µm diameter; cuboidal epithelium; CK7/CK19 positive)

7. Stroma (Extracellular Matrix)

Glisson capsule: dense hypocellular collagen encapsulating the entire liver; extends into portal tracts
Reticulin framework (types I and III collagen > 95% of total): demonstrated by reticulin stain - useful for detecting architectural distortion in pathology
Type IV collagen, laminin, fibronectin, and elastin complete the matrix

Key clinical correlations for exam:

Zone 3 necrosis: ischemia, paracetamol toxicity, CCl4
Zone 1 necrosis: phosphorus poisoning, eclampsia
Interface hepatitis (breach of limiting plate): autoimmune hepatitis, chronic viral hepatitis
Stellate cell activation: key step in hepatic fibrosis/cirrhosis
Canals of Hering: site of hepatic progenitor cells; proliferate in ductular reaction

Sources: Yamada's Textbook of Gastroenterology, 7th ed. (pp. 239-244); Sleisenger & Fordtran's GI and Liver Disease

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