histology Histology, also known as microscopic anatomy or microanatomy, is the branch of biology which studies the microscopic anatomy of biological tissues. Histology is the microscopic counterpart to gross anatomy, which looks at larger structures vis ...

(microscopic

anatomy Anatomy () is the branch of biology concerned with the study of the structure of organisms and their parts. Anatomy is a branch of natural science that deals with the structural organization of living things. It is an old science, having it ...

), 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 A capillary is a small blood vessel from 5 to 10 micrometres (μm) in diameter. Capillaries are composed of only the tunica intima, consisting of a thin wall of simple squamous endothelial cells. They are the smallest blood vessels in the body: ...

network, and a central vein. Lobules are different from the

lobes of liver In human anatomy, the liver is divided grossly into four parts or lobes: the right lobe, the left lobe, the caudate lobe, and the quadrate lobe. Seen from the front – the diaphragmatic surface - the liver is divided into two lobes: the right ...

: 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 th ...

(terminal acinus). Each zone is centered on the line connecting two portal triads 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 The portal vein or hepatic portal vein (HPV) is a blood vessel that carries blood from the gastrointestinal tract, gallbladder, pancreas and spleen to the liver. This blood contains nutrients and toxins extracted from digested contents. Appr ...

, a venule branch of the portal vein, with blood rich in nutrients but low in oxygen * one or two small

bile duct A bile duct is any of a number of long tube-like structures that carry bile, and is present in most vertebrates. Bile is required for the digestion of food and is secreted by the liver into passages that carry bile toward the hepatic duct. ...

ules 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 The hepatoduodenal ligament is the portion of the lesser omentum extending between the porta hepatis of the liver and the superior part of the duodenum. Running inside it are the following structures collectively known as the portal triad: * he ...

, 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 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 portal triad constituents.

Function

Zones differ by function: * zone I hepatocytes are specialized for oxidative liver functions such as gluconeogenesis, β-oxidation of fatty acids and cholesterol synthesis * 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 s ...

and

cytochrome P-450 Cytochromes P450 (CYPs) are a superfamily of enzymes containing heme as a cofactor that functions as monooxygenases. In mammals, these proteins oxidize steroids, fatty acids, and xenobiotics, and are important for the clearance of various comp ...

-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 NAPQI, also known as NAPBQI or ''N''-acetyl-''p''-benzoquinone imine, is a toxic byproduct produced during the xenobiotic metabolism of the analgesic paracetamol (acetaminophen). It is normally produced only in small amounts, and then almost imme ...

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 in hemochromatosis and zone II necrosis in yellow fever.

Clinical significance

Bridging fibrosis, a type of

fibrosis Fibrosis, also known as fibrotic scarring, is a pathological wound healing in which connective tissue replaces normal parenchymal tissue to the extent that it goes unchecked, leading to considerable tissue remodelling and the formation of perma ...

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