In histology (microscopic anatomy), the lobules of liver, or hepatic lobules, are small divisions of the liver defined at the microscopic scale. The hepatic lobule is a building block of the liver tissue, consisting of a portal triad, hepatocytes arranged in linear cords between a capillary network, and a central vein. Lobules are different from the lobes of liver: they are the smaller divisions of the lobes. The two-dimensional microarchitecture of the liver can be viewed from different perspectives: The term "hepatic lobule", without qualification, typically refers to the classical lobule.

Structure

The hepatic lobule can be described in terms of metabolic "zones", describing the hepatic

acinus An acinus (; plural, acini; adjective, acinar or acinous) refers to any cluster of cells that resembles a many-lobed "berry," such as a raspberry ('' acinus'' is Latin for "berry"). The berry-shaped termination of an exocrine gland, where the ...

(terminal acinus). Each zone is centered on the line connecting two

portal triad In histology (microscopic anatomy), the lobules of liver, or hepatic lobules, are small divisions of the liver defined at the microscopic scale. The hepatic lobule is a building block of the liver tissue, consisting of a portal triad, hepatocyte ...

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, making it least sensitive to ischemic injury while making it very susceptible to viral hepatitis. Conversely, the centrilobular zone III has the poorest oxygenation, and will be most affected during a time of ischemia.

Portal triad

A portal triad (also known as portal canal, portal field, portal area, or portal tract) is a distinctive arrangement within lobules. It consists of the following five structures: *

proper hepatic artery The hepatic artery proper (also proper hepatic artery) is the artery that supplies the liver and gallbladder. It raises from the common hepatic artery, a branch of the celiac artery. Structure The hepatic artery proper arises from the common h ...

, an arteriole branch of the hepatic artery that supplies oxygen * hepatic portal vein, a venule branch of the portal vein, with blood rich in nutrients but low in oxygen * one or two small bile ductules of cuboidal epithelium, branches of the

bile Bile (from Latin ''bilis''), or gall, is a dark-green-to-yellowish-brown fluid produced by the liver of most vertebrates that aids the digestion of lipids in the small intestine. In humans, bile is produced continuously by the liver (liver bile ...

conducting system. * lymphatic vessels * branch of the vagus nerve The misnomer "portal triad" traditionally has included only the first three structures, and was named before lymphatic vessels were discovered in the structure. It can refer both to the largest branch of each of these vessels running inside the hepatoduodenal ligament, and to the smaller branches of these vessels inside the liver. In the smaller portal triads, the four vessels lie in a network of

connective tissue Connective tissue is one of the four primary types of animal tissue, along with epithelial tissue, muscle tissue, and nervous tissue. It develops from the mesenchyme derived from the mesoderm the middle embryonic germ layer. Connective tiss ...

and are surrounded on all sides by hepatocytes. The ring of hepatocytes abutting the connective tissue of the triad is called the periportal limiting plate. File:Portal triad.JPG, Portal triad File:Gray1093.png, Labeled sketch of a portal canal File:Leber Glisson (Ratte).jpg, Portal triad of a rat liver, ''1'' branch of hepatic artery, ''2'' branch of portal vein, ''3'' bile duct File:Liver portal triad.png, Portal triad of mouse liver. 1= bile duct, 2= branch of hepatic artery, 3= branch of portal vein, 4= lymphatic vessels

Periportal space

The periportal space ( la, spatium periportale), or periportal space of Mall, is a space between the stroma of the portal canal and the outermost hepatocytes in the hepatic lobule, and is thought to be one of the sites where lymph originates in the liver. Fluid (residual blood plasma) that is not taken up by hepatocytes drains into the periportal space, and is taken up by the lymphatic vessels that accompany the other

constituents.

Function

Zones differ by function: * zone I hepatocytes are specialized for oxidative liver functions such as

gluconeogenesis Gluconeogenesis (GNG) is a metabolic pathway that results in the generation of glucose from certain non-carbohydrate carbon substrates. It is a ubiquitous process, present in plants, animals, fungi, bacteria, and other microorganisms. In vertebrat ...

, β-oxidation of fatty acids and

cholesterol synthesis The mevalonate pathway, also known as the isoprenoid pathway or HMG-CoA reductase pathway is an essential metabolic pathway present in eukaryotes, archaea, and some bacteria. The pathway produces two five-carbon building blocks called isopenten ...

* zone III cells are more important for

glycolysis Glycolysis is the metabolic pathway that converts glucose () into pyruvate (). The free energy released in this process is used to form the high-energy molecules adenosine triphosphate (ATP) and reduced nicotinamide adenine dinucleotide (NADH ...

lipogenesis In biochemistry, lipogenesis is the conversion of fatty acids and glycerol into fats, or a metabolic process through which acetyl-CoA is converted to triglyceride for storage in fat. Lipogenesis encompasses both fatty acid and triglyceride syn ...

and cytochrome P-450-based drug detoxification. This specialization is reflected histologically; the detoxifying zone III cells have the highest concentration of

CYP2E1 Cytochrome P450 2E1 (abbreviated CYP2E1, ) is a member of the cytochrome P450 mixed-function oxidase system, which is involved in the metabolism of xenobiotics in the body. This class of enzymes is divided up into a number of subcategories, includ ...

and thus are most sensitive to NAPQI production in

acetaminophen Paracetamol, also known as acetaminophen, is a medication used to treat fever and mild to moderate pain. Common brand names include Tylenol and Panadol. At a standard dose, paracetamol only slightly decreases body temperature; it is inferior ...

toxicity. Other zonal injury patterns include zone I deposition of

hemosiderin Hemosiderin image of a kidney viewed under a microscope. The brown areas represent hemosiderin Hemosiderin or haemosiderin is an iron-storage complex that is composed of partially digested ferritin and lysosomes. The breakdown of heme gives rise ...

in hemochromatosis and zone II necrosis in yellow fever.

Clinical significance

Bridging fibrosis, a type of fibrosis seen in several types of liver injury, describes fibrosis from the central vein to the portal triad.

References

External links

Histology at siumed.edu

* {{DEFAULTSORT:Lobules Of Liver Liver anatomy