Hepatic lobule
Encyclopedia
A hepatic lobule is a small division of the liver
defined at the histological scale. It should not be confused with the anatomic lobes of the liver (caudate lobe, quadrate lobe, left lobe, and right lobe), or any of the functional lobe classification systems.
The two-dimensional microarchitecture of the liver can be viewed from multiple different perspectives:
The term "hepatic lobule", without qualification, typically refers to the classical lobule.
s and extends outwards to the two adjacent central veins. The periportal zone I is nearest to the entering vascular supply and receives the most oxygenated blood. Conversely, the centrilobular zone III has the poorest oxygenation and is relatively more sensitive to ischemic injury .
Functionally, zone I hepatocytes are specialized for oxidative liver functions such as gluconeogenesis
, β-oxidation of fatty acids and cholesterol synthesis, while zone III cells are more important for glycolysis
, lipogenesis
and cytochrome P-450-based drug detoxification . This specialization is reflected histologically; the detoxifying zone III cells have the highest concentration of CYP2E1
and thus are most sensitive to NAPQI
production in acetaminophen toxicity . Other zonal injury patterns include zone I deposition of hemosiderin
in hemochromatosis and zone II necrosis in yellow fever
.
Liver
The liver is a vital organ present in vertebrates and some other animals. It has a wide range of functions, including detoxification, protein synthesis, and production of biochemicals necessary for digestion...
defined at the histological scale. It should not be confused with the anatomic lobes of the liver (caudate lobe, quadrate lobe, left lobe, and right lobe), or any of the functional lobe classification systems.
The two-dimensional microarchitecture of the liver can be viewed from multiple different perspectives:
| Name | Shape | Model >- | classical lobule |
hexagonal; divided into concentric centrilobular, midzonal, periportal parts | >- | triangular; centered around a portal triad Portal triad A portal triad is a distinctive arrangement in the liver. It is a component of the hepatic lobule. It consists of the following five structures:* hepatic artery* hepatic portal vein* bile duct*lymphatic vessels... |
>- | elliptical or diamond-shaped; divided into zone I (periportal), zone II (transition zone), and zone III (centrilobular) | blood flow and metabolic |
The term "hepatic lobule", without qualification, typically refers to the classical lobule.
Zones
From a metabolic perspective, the functional unit is the hepatic acinus (terminal acinus), each of which is centered on the line connecting two portal triadPortal triad
A portal triad is a distinctive arrangement in the liver. It is a component of the hepatic lobule. It consists of the following five structures:* hepatic artery* hepatic portal vein* bile duct*lymphatic vessels...
s and extends outwards to the two adjacent central veins. The periportal zone I is nearest to the entering vascular supply and receives the most oxygenated blood. Conversely, the centrilobular zone III has the poorest oxygenation and is relatively more sensitive to ischemic injury .
Functionally, zone I hepatocytes are specialized for oxidative liver functions such as gluconeogenesis
Gluconeogenesis
Gluconeogenesis is a metabolic pathway that results in the generation of glucose from non-carbohydrate carbon substrates such as lactate, glycerol, and glucogenic amino acids....
, β-oxidation of fatty acids and cholesterol synthesis, while zone III cells are more important for glycolysis
Glycolysis
Glycolysis is the metabolic pathway that converts glucose C6H12O6, into pyruvate, CH3COCOO− + H+...
, lipogenesis
Lipogenesis
Lipogenesis is the process by which acetyl-CoA is converted to fats. The former is an intermediate stage in metabolism of simple sugars, such as glucose, a source of energy of living organisms. Through lipogenesis, the energy can be efficiently stored in the form of fats...
and cytochrome P-450-based drug detoxification . This specialization is reflected histologically; the detoxifying zone III cells have the highest concentration of CYP2E1
CYP2E1
Cytochrome P450 2E1 , a member of the cytochrome P450 mixed-function oxidase system, is involved in the metabolism of xenobiotics in the body. In humans, the CYP2E1 enzyme is encoded by the CYP2E1 gene...
and thus are most sensitive to NAPQI
NAPQI
NAPQI is a toxic byproduct produced during the xenobiotic metabolism of the analgesic paracetamol...
production in acetaminophen toxicity . Other zonal injury patterns include zone I deposition of hemosiderin
Hemosiderin
thumb|Hemosiderin image of a kidney viewed under a microscope. The brown areas represent hemosiderinHemosiderin or haemosiderin is an iron-storage complex. It is always found within cells and appears to be a complex of ferritin, denatured ferritin and other material...
in hemochromatosis and zone II necrosis in yellow fever
Yellow fever
Yellow fever is an acute viral hemorrhagic disease. The virus is a 40 to 50 nm enveloped RNA virus with positive sense of the Flaviviridae family....
.