SN1 SHORT NOTE 5M ★ High-yield

Describe the histology of the liver with its lobule and portal triad.

Anatomy · 1st Professional — Pre-Clinical

The liver is organised into hepatic lobules, roughly hexagonal units with a central vein and portal triads at the angles.

Hepatocytes are arranged in plates one cell thick, radiating from the central vein like the spokes of a wheel
Between the plates lie the sinusoids
In man the lobules are poorly demarcated, because the interlobular connective tissue is scanty; in the pig they are clearly outlined

Component	Features
Branch of the portal vein	The largest; thin-walled, wide, irregular lumen
Branch of the hepatic artery	Small, thick muscular wall, round lumen
Interlobular bile duct	Lined by cuboidal or columnar epithelium — the only one of the three with an epithelial lining
Lymphatic and nerve	Small, often not seen

Correctly a "portal tract", since it contains more than three structures
The bile duct is identified by its epithelium, which is how the triad is oriented in a section

Sinusoids — wide capillaries with a discontinuous, fenestrated endothelium and NO basement membrane, so plasma reaches the hepatocyte directly
Kupffer cells — fixed macrophages within the sinusoidal lumen; phagocytose bacteria and aged red cells
Space OF DISSE — the perisinusoidal space between the endothelium and the hepatocyte, containing microvilli and lymph
Hepatic stellate (Ito) cells in the space of Disse store vitamin A, and transform into myofibroblasts that lay down the collagen of cirrhosis

Concept	Centre	Significance
Classical lobule	Central vein	Structural and descriptive
Portal lobule	Portal triad	Biliary drainage
Acinus of Rappaport	A diamond between two central veins	Functional — explains the pattern of injury

Zone 1 (periportal) — best oxygenated; first affected by toxins arriving in the blood and by viral hepatitis
Zone 3 (centrilobular) — least oxygenated; first affected by ischaemia, shock and paracetamol, giving centrilobular necrosis

Cirrhosis — diffuse fibrosis with regenerative nodules; the normal lobular architecture is destroyed, which is what raises portal pressure
Ito cells are the target of anti-fibrotic research, since their activation is the central event in liver fibrosis
"Nutmeg liver" of chronic venous congestion — centrilobular congestion and necrosis alternating with paler periportal tissue
Liver biopsy is assessed for both grade (activity) and stage (fibrosis), which determine treatment in chronic hepatitis
The absence of a sinusoidal basement membrane is what allows such free exchange with the hepatocyte — and its formation ("capillarisation") in cirrhosis impairs liver function further

All questions in Systemic Histology