Hepatic Lobule
Hexagonal prism (~1–2 mm diameter, 1.5–2 mm length). Central vein (terminal hepatic venule) at axis; 6 portal triads (portal venule + hepatic arteriole + bile duct) at vertices. Hepatocyte cords radiate from triads toward the central vein. Sinusoids lined by fenestrated LSECs and Kupffer cells; Space of Disse enables direct plasma–hepatocyte exchange. Metabolic zonation along the O₂ gradient: Zone 1 (gluconeogenesis, β-oxidation) → Zone 3 (CYP450, lipogenesis, ischemia-prone).
Overview
The hepatic lobule is the fundamental architectural and functional unit of the liver, the repeating structural module around which the liver's remarkable metabolic capacity is organised. On cross-section it appears as a hexagonal prism approximately 1–2 mm in diameter and 1.5–2 mm in length. At the geometric centre lies the central vein (a tributary of the hepatic vein draining into the IVC); at each of the six corners sits a portal triad containing a portal venule, a hepatic arteriole, and a bile duct.
An adult human liver contains approximately 100,000 hepatic lobules. These functional units are not sharply demarcated by connective tissue septa in humans (unlike pigs or pathologic congestive hepatopathy), but their boundaries are functionally defined by vascular territory. The lobular organisation explains why acetaminophen and ischemic injury preferentially destroy zone-3 (centrilobular) hepatocytes, why certain viral hepatitides cause periportal necrosis, and how the portal circulation creates the first-pass metabolic barrier that processes every nutrient, drug, and toxin from the gut before systemic exposure.
Structure — Blood and Bile Flow
PORTAL TRIAD (x6 per lobule — hexagon vertices)
+--------------------------------------------------+
| Portal venule [nutrient-rich, O2-depleted] |
| Hepatic arteriole [O2-rich, from celiac trunk] |
| Bile duct [cholangiocytes, HCO3- secretion] |
| Lymphatics (larger triads) + nerve fibres |
+------------------+-------------------------------+
| mixed blood enters sinusoids
v
==========================================
HEPATIC SINUSOIDS (7-15 um wide)
------------------------------------------
LSEC (fenestrated, 100-150 nm pores, no diaphragm,
no basement membrane below them)
<-- plasma filtration through fenestrae -->
SPACE OF DISSE (0.2-0.5 um perisinusoidal space)
<-- direct plasma <-> hepatocyte exchange -->
HEPATOCYTE CORDS (1-2 cells thick, radially arranged)
==========================================
| BLOOD FLOWS CENTRIPETALLY
v
CENTRAL VEIN (terminal hepatic venule, ~50-80 um)
|
v
Sublobular veins --> Hepatic veins --> IVC
BILE FLOWS COUNTER-CURRENT (periphery to portal tract):
Bile canaliculi (~1 um, between hepatocyte apical surfaces)
--> Canal of Hering (hepatocyte/cholangiocyte junction)
--> Bile ductules --> Portal bile duct
--> Intrahepatic biliary tree --> CBD --> duodenum
Non-Parenchymal Cell Types
| Cell Type | Location | Function |
|---|---|---|
| LSECs (liver sinusoidal endothelial cells) | Sinusoid lining | Fenestrated (100–150 nm, no diaphragm, no basement membrane); filter plasma to Disse space; scavenge oxidised LDL, colloidal waste; produce HGF, VEGF; modulate fibrosis via LSEC capillarisation in cirrhosis |
| Kupffer cells | Sinusoid lumen, adherent to LSEC | Largest tissue macrophage population in the body (yolk sac + BM origin); phagocytose gut-derived bacteria, LPS, aged RBCs; TLR4/LPS → TNF-α, IL-6, IL-1β → hepatocyte acute-phase response and fibrogenic signalling |
| Hepatic stellate cells (HSC / Ito cells) | Space of Disse | Quiescent: store vitamin A as retinyl esters (80% total body retinol); GFAP+, desmin+. Activated by TGF-β1/PDGF: lose retinol, upregulate α-SMA, PDGFRβ → collagen I/III secretion → perisinusoidal fibrosis → cirrhosis |
| Pit cells (liver NK cells) | Sinusoid lumen | Liver-resident CD56+ NK cells; surveillance against virus-infected and malignant hepatocytes via perforin/granzyme and TRAIL pathways |
| Cholangiocytes | Bile duct epithelium | Line interlobular and septal bile ducts; HCO₃⁻ secretion (CFTR/AE2); water modification via AQP1; primary autoimmune targets in PBC (AMA: anti-PDC-E2) and PSC |
Metabolic Zonation — Rappaport Hepatic Acinus
The Rappaport acinus model describes metabolic zones along the sinusoidal axis from a portal triad blood inlet to the central vein. The pO₂ gradient (~65–70 mmHg zone 1 → ~35 mmHg zone 3) drives zone-specific enzyme expression, dynamically maintained by Wnt/EGF morphogen gradients.
Zone 1 — Periportal (pO₂ ~70 mmHg, O₂-rich)
Specialises in: gluconeogenesis (PEPCK, PC), β-oxidation, urea synthesis (CPS-I), primary bile acid secretion via NTCP, amino acid deamination, glutamate dehydrogenase. Resistant to ischemia; target of periportal hepatitis (viral hepatitis B, yellow fever, AIH interface hepatitis).
Zone 3 — Centrilobular (pO₂ ~35 mmHg, O₂-poor)
Specialises in: glycolysis, de novo lipogenesis (ChREBP, SREBP1c), CYP3A4/CYP2E1 drug metabolism (highest expression here), glutamine synthesis (GS — reliable zone-3 IHC marker), ketogenesis. Most vulnerable to ischemia, acetaminophen toxicity, ethanol-induced injury, and congestive hepatopathy.
Zonation is actively maintained: Wnt3a/Wnt2 secreted from central vein endothelium keeps zone-3 hepatocytes in the perivenous fate (GS, CYP2E1, β-catenin-high programme). EGF/EGFR signalling from portal tract maintains periportal fate (PEPCK, E-cadherin). Disruption of these morphogen gradients in NASH, cirrhosis, or hepatic regeneration causes metabolic de-zonation.
| Model | Unit Definition | Primary Utility |
|---|---|---|
| Classic lobule | Hexagonal; central vein centre; portal triads at vertices | Histological description; pathologist's model for grading/staging |
| Portal lobule | Triangular; portal triad centre; bile flow orientation | Biliary pathology (PBC, PSC, biliary obstruction) |
| Hepatic acinus (Rappaport) | Diamond/rhomboid; zones 1–3 along sinusoid axis | Clinically most useful; explains ischemic, toxic, and metabolic injury patterns |
Function — First-Pass Metabolism
Every nutrient absorbed by the gut epithelium enters the portal circulation and must pass through the hepatic lobule before reaching systemic blood. This portal first-pass creates a regulated buffer:
| Substrate | Zone 1 Processing | Clinical Consequence |
|---|---|---|
| Glucose | Extracted by GLUT2/glucokinase → glycogen synthesis (postprandial); surplus → de novo lipogenesis (zone 3) → VLDL | Hepatic glucose output impaired in cirrhosis and acute liver failure |
| Amino acids | Deamination; nitrogen → urea cycle (zone 1 CPS-I); carbon → gluconeogenesis or Krebs | Hyperammonemia in liver failure; hepatic encephalopathy |
| Bile acids | >95% portal bile acids reabsorbed by NTCP (zone 1) — enterohepatic circulation efficiency | Bile acid malabsorption in ileal disease; primary biliary cholangitis disrupts NTCP |
| Drugs / xenobiotics | Phase I CYP450 (zone 3 dominant for CYP2E1/3A4); Phase II conjugation; Phase III export | First-pass metabolism limits oral bioavailability; polymorphisms create drug toxicity risk |
| LPS / microbiome signals | Kupffer cells phagocytose and pattern-recognise via TLR4; prevent systemic bacteraemia | Gut-liver axis; leaky gut in cirrhosis → Kupffer overactivation → NASH progression |
Pathology
Centrilobular Necrosis
Zone-3 hepatocyte death from ischemia (right heart failure / congestive hepatopathy, shock, Budd-Chiari syndrome) or zone-3-specific toxins. Acetaminophen: CYP2E1 → NAPQI → GSH depletion → zone-3 necrosis. Carbon tetrachloride: CYP2E1 → CCl₃• radical → lipid peroxidation. Histology: zone-3 coagulative necrosis, sinusoidal dilation. Gross pathology: "nutmeg liver" in congestive hepatopathy.
Hepatic Fibrosis and Cirrhosis
Persistent injury (NASH, viral hepatitis, alcohol, biliary) → hepatocyte death → TGF-β1 + PDGF release → HSC activation → α-SMA+ myofibroblasts → perisinusoidal collagen I/III deposition (pericellular/chicken-wire fibrosis in NASH; portal–bridging fibrosis in viral) → architectural distortion → nodular regeneration → cirrhosis. Portal hypertension results from increased sinusoidal resistance (fibrosis + activated HSC contraction).
Intrahepatic Cholestasis
Failure of canalicular bile flow: DILI (chlorpromazine, oestrogen, amoxicillin-clavulanate), viral hepatitis, PFIC (BSEP/MDR3/FIC1 mutations), ICP (intrahepatic cholestasis of pregnancy — ABCB4 mutations). Conjugated hyperbilirubinemia; bile acid accumulation → hepatocyte toxicity; pruritus from bile acid tissue deposition. Histology: canalicular bile plugs, periportal cholate stasis.
Periportal Interface Hepatitis
Destruction of zone-1 hepatocytes at the limiting plate (portal–parenchymal interface) by lymphoplasmacytic infiltrate. Hallmark of autoimmune hepatitis (AIH; anti-smooth muscle Ab, anti-LKM1), chronic viral hepatitis B/C, and drug reactions. Graded A1–A3 (Metavir). Severe interface hepatitis is the major determinant of fibrosis progression speed.
Sinusoidal Obstruction Syndrome (SOS/VOD)
HSCT preparative conditioning (cyclophosphamide, busulfan, gemtuzumab ozogamicin) → LSEC injury → fibrin/collagen deposition in sinusoids → hepatic venule occlusion → portal hypertension → ascites, hepatomegaly, multi-organ failure. Defibrotide (approved EU/US) reduces SOS mortality. LSEC capillarisation (loss of fenestrae, BM deposition) is also an early event in cirrhogenesis, impairs hepatocyte–plasma exchange, and worsens metabolic zonation.
Cross-Atlas Connections
References
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