A hepatocyte is a cell of the main

parenchyma Parenchyma () is the bulk of functional substance in an animal organ or structure such as a tumour. In zoology it is the name for the tissue that fills the interior of flatworms. Etymology The term ''parenchyma'' is New Latin from the word π ...

l tissue of the liver. Hepatocytes make up 80% of the liver's mass. These cells are involved in: *

Protein synthesis Protein biosynthesis (or protein synthesis) is a core biological process, occurring inside Cell (biology), cells, homeostasis, balancing the loss of cellular proteins (via Proteolysis, degradation or Protein targeting, export) through the product ...

* Protein storage * Transformation of carbohydrates * Synthesis of cholesterol, bile salts and

phospholipid Phospholipids, are a class of lipids whose molecule has a hydrophilic "head" containing a phosphate group and two hydrophobic "tails" derived from fatty acids, joined by an alcohol residue (usually a glycerol molecule). Marine phospholipids typ ...

s * Detoxification, modification, and excretion of exogenous and endogenous substances * Initiation of formation and secretion of

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 ...

Structure

The typical hepatocyte is cubical with sides of 20-30

μm The micrometre ( international spelling as used by the International Bureau of Weights and Measures; SI symbol: μm) or micrometer (American spelling), also commonly known as a micron, is a unit of length in the International System of Unit ...

, (in comparison, a human hair has a diameter of 17 to 180 μm).The

diameter In geometry, a diameter of a circle is any straight line segment that passes through the center of the circle and whose endpoints lie on the circle. It can also be defined as the longest chord of the circle. Both definitions are also valid fo ...

of human hair ranges from 17 to 181 μm. The typical volume of a hepatocyte is 3.4 x 10⁻⁹ cm³. Smooth

endoplasmic reticulum The endoplasmic reticulum (ER) is, in essence, the transportation system of the eukaryotic cell, and has many other important functions such as protein folding. It is a type of organelle made up of two subunits – rough endoplasmic reticulum ( ...

is abundant in hepatocytes, in contrast to most other cell types.

Microanatomy

Hepatocytes display an eosinophilic cytoplasm, reflecting numerous

mitochondria A mitochondrion (; ) is an organelle found in the Cell (biology), cells of most Eukaryotes, such as animals, plants and Fungus, fungi. Mitochondria have a double lipid bilayer, membrane structure and use aerobic respiration to generate adenosi ...

, and basophilic stippling due to large amounts of smooth endoplasmic reticulum and free ribosomes. Brown lipofuscin granules are also observed (with increasing age) together with irregular unstained areas of cytoplasm; these correspond to cytoplasmic

glycogen Glycogen is a multibranched polysaccharide of glucose that serves as a form of energy storage in animals, fungi, and bacteria. The polysaccharide structure represents the main storage form of glucose in the body. Glycogen functions as one o ...

and lipid stores removed during histological preparation. The average life span of the hepatocyte is 5 months; they are able to regenerate. Hepatocyte nuclei are round with dispersed chromatin and prominent nucleoli. Anisokaryosis (or variation in the size of the nuclei) is common and often reflects tetraploidy and other degrees of polyploidy, a normal feature of 30-40% of hepatocytes in the adult human liver. Binucleate cells are also common. Hepatocytes are organised into plates separated by vascular channels ( sinusoids), an arrangement supported by a reticulin (

collagen type III Collagen () is the main structural protein in the extracellular matrix found in the body's various connective tissues. As the main component of connective tissue, it is the most abundant protein in mammals, making up from 25% to 35% of the whole ...

) network. The hepatocyte plates are one cell thick in mammals and two cells thick in the chicken. Sinusoids display a discontinuous, fenestrated endothelial cell lining. The endothelial cells have no

basement membrane The basement membrane is a thin, pliable sheet-like type of extracellular matrix that provides cell and tissue support and acts as a platform for complex signalling. The basement membrane sits between Epithelium, epithelial tissues including mesot ...

and are separated from the hepatocytes by the space of Disse, which drains lymph into the portal tract lymphatics. Kupffer cells are scattered between endothelial cells; they are part of the reticuloendothelial system and phagocytose spent erythrocytes. Stellate (Ito) cells store vitamin A and produce extracellular matrix and

collagen Collagen () is the main structural protein in the extracellular matrix found in the body's various connective tissues. As the main component of connective tissue, it is the most abundant protein in mammals, making up from 25% to 35% of the who ...

; they are also distributed amongst endothelial cells but are difficult to visualise by light microscopy.

Function

Protein synthesis

The hepatocyte is a cell in the body that manufactures serum albumin, fibrinogen, and the prothrombin group of

clotting factor Coagulation, also known as clotting, is the process by which blood changes from a liquid to a gel, forming a blood clot. It potentially results in hemostasis, the cessation of blood loss from a damaged vessel, followed by repair. The mechanism o ...

s (except for Factors 3 and 4). It is the main site for the synthesis of lipoproteins, ceruloplasmin, transferrin, complement, and

glycoprotein Glycoproteins are proteins which contain oligosaccharide chains covalently attached to amino acid side-chains. The carbohydrate is attached to the protein in a cotranslational or posttranslational modification. This process is known as glycos ...

s. Hepatocytes manufacture their own structural proteins and intracellular enzymes. Synthesis of proteins is by the rough endoplasmic reticulum (RER), and both the rough and smooth endoplasmic reticulum (SER) are involved in secretion of the proteins formed. The

(ER) is involved in conjugation of proteins to lipid and carbohydrate moieties synthesized by, or modified within, the hepatocytes. Proteins produced by hepatocytes that function as hormones are known as hepatokines.

Carbohydrate metabolism

The liver forms fatty acids from carbohydrates and synthesizes triglycerides from fatty acids and glycerol. Hepatocytes also synthesize

apoprotein Apoprotein may refer to: * Apoenzyme, the protein part of an enzyme without its characteristic prosthetic group * Apolipoprotein, a lipid-binding protein that is a constituent of the plasma lipoprotein {{disambiguation ...

s with which they then assemble and export lipoproteins ( VLDL, HDL). The liver is also the main site in the body for

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 ...

, the formation of carbohydrates from precursors such as alanine, glycerol, and oxaloacetate.

Lipid metabolism

The liver receives many lipids from the systemic circulation and metabolizes chylomicron remnants. It also synthesizes cholesterol from

acetate An acetate is a salt (chemistry), salt formed by the combination of acetic acid with a base (e.g. Alkali metal, alkaline, Alkaline earth metal, earthy, Transition metal, metallic, nonmetallic or radical Radical (chemistry), base). "Acetate" als ...

and further synthesizes bile salts. The liver is the sole site of bile salts formation.

Detoxification

Hepatocytes have the ability to metabolize, detoxify, and inactivate exogenous compounds such as

drugs A drug is any chemical substance that causes a change in an organism's physiology or psychology when consumed. Drugs are typically distinguished from food and substances that provide nutritional support. Consumption of drugs can be via inhalat ...

(see drug metabolism),

insecticide Insecticides are substances used to kill insects. They include ovicides and larvicides used against insect eggs and larvae, respectively. Insecticides are used in agriculture, medicine, industry and by consumers. Insecticides are claimed to b ...

s, and endogenous compounds such as

steroid A steroid is a biologically active organic compound with four rings arranged in a specific molecular configuration. Steroids have two principal biological functions: as important components of cell membranes that alter membrane fluidity; and a ...

s. The drainage of the intestinal venous blood into the liver requires efficient detoxification of miscellaneous absorbed substances to maintain homeostasis and protect the body against ingested toxins. One of the detoxifying functions of hepatocytes is to modify ammonia into urea for excretion. The most abundant organelle in liver cells is the smooth endoplasmic reticulum.

Society and culture

Use in research

Primary hepatocytes are commonly used in cell biological and biopharmaceutical research. In vitro model systems based on hepatocytes have been of great help to better understand the role of hepatocytes in (patho)physiological processes of the liver. In addition, pharmaceutical industry has heavily relied on the use of hepatocytes in suspension or culture to explore mechanisms of drug metabolism and even predict in vivo drug metabolism. For these purposes, hepatocytes are usually isolated from animal or human whole liver or liver tissue by collagenase digestion, which is a two-step process. In the first step, the liver is placed in an isotonic solution, in which calcium is removed to disrupt cell-cell tight junctions by the use of a calcium chelating agent. Next, a solution containing collagenase is added to separate the hepatocytes from the liver stroma. This process creates a suspension of hepatocytes, which can be seeded in multi-well plates and cultured for many days or even weeks. For optimal results, culture plates should first be coated with an extracellular matrix (e.g. collagen, Matrigel) to promote hepatocyte attachment (typically within 1-3 hr after seeding) and maintenance of the hepatic phenotype. In addition, and overlay with an additional layer of extracellular matrix is often performed to establish a sandwich culture of hepatocytes. The application of a sandwich configuration supports prolonged maintenance of hepatocytes in culture. Freshly-isolated hepatocytes that are not used immediately can be cryopreserved and stored. They do not proliferate in culture. Hepatocytes are intensely sensitive to damage during the cycles of cryopreservation including freezing and thawing. Even after the addition of classical cryoprotectants there is still damage done while being cryopreserved. Nevertheless, recent cryopreservation and resuscitation protocols support application of cryopreserved hepatocytes for most biopharmaceutical applications.

Additional images

File:Hepato-biliary.jpg, Schemic diagram of Biliary system

References

External links

* - "Ultrastructure of the Cell: hepatocytes and sinusoids"
Hepatic Histology: Hepatocytes (Colorado State University
{{Authority control Liver anatomy Animal cells Human cells