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 vi ...

(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 The liver is a major organ only found in vertebrates which performs many essential biological functions such as detoxification of the organism, and the synthesis of proteins and biochemicals necessary for digestion and growth. In humans, it ...

defined at the microscopic scale. The hepatic lobule is a building block of the liver tissue, consisting of a portal triad,

hepatocyte A hepatocyte is a cell of the main parenchymal tissue of the liver. Hepatocytes make up 80% of the liver's mass. These cells are involved in: * Protein synthesis * Protein storage * Transformation of carbohydrates * Synthesis of cholesterol, ...

s 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 bod ...

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 (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, 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. Approx ...

, 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. It ...

ules of cuboidal epithelium, branches of the bile conducting system. *

lymphatic vessels The lymphatic vessels (or lymph vessels or lymphatics) are thin-walled vessels (tubes), structured like blood vessels, that carry lymph. As part of the lymphatic system, lymph vessels are complementary to the cardiovascular system. Lymph vessel ...

* branch of the

vagus nerve The vagus nerve, also known as the tenth cranial nerve, cranial nerve X, or simply CN X, is a cranial nerve that interfaces with the parasympathetic control of the heart, lungs, and digestive tract. It comprises two nerves—the left and righ ...

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: * hep ...

, 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

s. 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

s in the hepatic lobule, and is thought to be one of the sites where

lymph Lymph (from Latin, , meaning "water") is the fluid that flows through the lymphatic system, a system composed of lymph vessels (channels) and intervening lymph nodes whose function, like the venous system, is to return fluid from the tissues ...

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, β-oxidation of fatty acids and cholesterol synthesis * zone III cells are more important for glycolysis, lipogenesis 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 compo ...

-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 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 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 ...

hemochromatosis Iron overload or hemochromatosis (also spelled ''haemochromatosis'' in British English) indicates increased total accumulation of iron in the body from any cause and resulting organ damage. The most important causes are hereditary haemochromatos ...

and zone II necrosis in

yellow fever Yellow fever is a viral disease of typically short duration. In most cases, symptoms include fever, chills, loss of appetite, nausea, muscle pains – particularly in the back – and headaches. Symptoms typically improve within five days. ...

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