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HEPATOCYTE

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1. Morphology
2. Functions

Hepatocytes are the cells of the liver, accounting for about 80 % of this organ. They are organized in sheets of about one cell in thickness. The sheets are connected between each other forming a spongy-like structure (Figures 1 and 2). Hepatocytes are rather long-living cells, being renewing about every 5 months. However, it may change under regenerative processes where hepatocytes show a high ability for proliferation and regeneration of damaged liver tissue.

Hepatocyte
Figure 1. Liver organization. In figure E, hepatocytes are magnified.
Hepatocitos
Figure 2. Binucleated hepatocytes and sinusoids with erythrocytes. White areas of the perypheric cytoplasm of hepatocytes are lipid depots.

1. Morphology

Hepatocytes are polyedral cells, that is, they have several faces. They usually show 6 faces, but the number may vary. Faces are in contact with either other hepatocyte or with a sinusoid (Figure 3). Hepatocytes are large cells, about 20 to 30 µm in diameter. They show a round nucleus centered in the cytoplasm. However, in liver of adult humans, up to 25 % of the hepatocytes may be binucleated (Figure 2). The majority of nuclei are tetraploids, so they contain double amount of DNA than an ordinary cell. The size of nuclei is variable, although they are larger in tetraploid cells. Nuclei show scattered heterochromatin and one or more nucleoli. It is not common to observe mitotic hepatocytes (1 every 10000 or 20000 hepatocytes) in normal conditions. However, the mitotic hepatocytes increase enormously under liver injuries and regeneration processes. The cytoplasm features varies according to the physiological state of the cell, mostly influenced by fat and glycogen depots. There are many small mitochondria, 800 to 100 per hepatocyte. It is estimated that one hepatocyte may contain about 50 Golgi apparatuses that are usually organized in stacks of 3 to 5 cisterns showing thickened lateral regions containing dark bodies. Cistern stacks are somehow scattered in the cytoplasm, although they are often observed close to billiary canaliculi (Figures 3 and 4). Hepatocytes contain many peroxisomes (200 to 300), more than other regular cells. Near the billiary canaliculi, many lysosomes are also found.

Hepatocito
Figure 3. Hepatocyte ultrastructure.
Billiary canaliculi
Figure 4. Scanning electron microscopy image showing sinusoids between hepatocyte layers. Bile canaliculi appear as irregular small channels formed by the hepatocyte plasma membranes.

Abundant glycogen and lipid depots are found in the hepatocyte cytoplasm (the granular appearance of hepatocytes after haematoxylin and eosin staining is due to the holes left by lipid extraction from the cytoplasm during the tissue processing). In the cytoplasm, there are also residual bodies containing lipofuscin. The smooth endoplasmic reticulum is quite abundant, although the organelle size varies with the metabolic activity of the hepatocyte. It is concentrated around glycogen depots. In the liver lobules, there are morphological differences when comparing peripheral and central hepatocytes, mostly influenced by the blood features. For example, after digestion, peripheral hepatocytes are the first to store glycogen, but the last in movilize this glycogen when the rest of the body is demanding it. However, fat storing happens first in the centrally located hepatocytes, which usually have more smooth endoplasmic reticulum. On the other hand, the rough endoplasmic reticulum shows 50 % more surface in the peripheral and medium located hepatocytes than those in the inner part of the liver lobule.

Unlike other epithelial cells, hepatocytes are not bound to a basal membrane. Their basolateral membranes are surrounded by a low-density extracellular matrix synthesized by the hepatocytes themselves. It facilitates the diffusion and exchange of molecules with the sinusoids through the space of Disse, or perisinousoidal spaces, which is the space between the fenestrated endothelium and the hepatocytes. This extracellular matrix lacks laminin, at least when the hepatocyte is differentiated. However, type IV collagen and fibronectin look necessary for a proper hepatocyte differentiation. Hepatocytes are connected between each other by gap junctions, adherent junctions, desmosomes and tight junctions.

Hepatocytes are polarized cells, that is, there are differences between the regions facing the bile canaliculi and the regions close to the sinusoids. The polarity is essential for the correct function of the hepatocyte, and it is disorganized in many liver pathologies. The apical region is in contact with bile canaliculi. As in apical domain of epithelial cells, there are tight junctions, which seal and maintain the integrity of bile canaliculi. The apical membrane folds in microvilli that enormously increase the membrane surface. The apical membrane is about 13 % of the total hepatocyte membrane, and it is able to contain a large amount of molecules. Removing tight junctions leads the cell polarity disorganization. Hepatocyte polarity and bile canaliculi are established during the embryo development period.

The functional polarity relies on an unequal distribution of transporters and other membrane molecules between the apical and the baso-lateral plasma membrane domains. ABC transporters (ATP binding cassettes) are among the most important apical transporters in hepatocytes. The Golgi apparatus, endosomes and cytoskeleton (microtubules and actin filaments) are responsible for the differential distribution of molecules between the two membrane domains. There are two delivering pathways of proteins to the apical domain (Figure 5): from the Golgi apparatus, the proteins (for example, ABC transporters) are released in vesicles toward the apical plasma membrane or toward recycling endosomes, which function as intermediaries. Other proteins follow a transcytosis pathway, traveling first to basolateral membranes and then they are enclosed in endocytosis vesicles toward endosomes, where they are again packaged vesicles and shipped to the apical membrane. More rare is a pathway involving the exocytosis of lysosomes, followed by cooper transporters.

Vesicular pathways
Figure 5. Vesicular pathways toward the apical plasma membrane. A: Transcytosis. Single-pass transmembrane proteins are included in vesicles in Golgi apparatus. These vesicles fuse with the basolateral membranes, and then proteins are included again in endocytic vesicles that fuse with endosomes. Proteins are finally included in endosomal vesicles toward the apical plasma membrane of the hepatocyte. B: Direct pathway. ABC transporters are directly shipped in vesicles from the Golgi apparatus to the apical plasma membrane. C: Intermediary endosomes. ABC transporters may also be first targeted to intermediary endosomes and then packaged in vesicles that travel from endosomes to the plasma membrane (Adapted from Gissen and Arias, 2015).

2. Functions

The main function of hepatocytes is to metabolize substances coming from digestion. The liver is irrigated by the portal vein that gathered molecules resulting from digestion in the intestine. Hepatocytes are also strongly involved in detoxification of potentially harmful molecules. On the other hand, hepatocytes synthesize bile, which is finally released into the intestine and helps in digestion. For both functions, metabolizing molecules from digestion and releasing bile, hepatocytes are placed in a privileged location: in contact with sinusoids, which bring intestine digested molecules, and form the bile canaliculi that drain the bile from the lobules of the liver.

Glucose levels. Hepatocytes fetch glucose molecules coming from digestion and store them as glycogen, which is mobilized when the body needs energy. Glycogen is commonly found near the endoplasmic reticulum since the glucose-6-phosphatase enzyme is located in this organelle. Glucose-6-phosphatase catalyzes glucose-6-phosphate, the molecular form of glucose after glycogen catabolism, and produces free glucose, which can exit the hepatocyte and reach the blood stream.

Molecule synthesis. Bile salts, that help with the digestion of fat, are one of the substances synthesized by the hepatocytes. In the smooth endoplasmic reticulum, there are many enzymes involved in the synthesis of cholesterol and other lipids. In addition, hepatocytes produce lipoproteins needed for the transport of lipids in the blood stream. Fibrinogen for blood clotting and plasma albumins are also synthesized by hepatocytes. In the liver, urea is produced as a byproduct of the protein degradation. The production and accumulation of high amount of urea in the organism may be harmful. Hepatocytes store vitamin A and B, and heparin.

Lipidic metabolism. Beta-oxidation, involved in lipid catabolism, is working in the abundant peroxisomes of hepatocytes.

Detoxification. Hepatocytes gather nutritious substances coming from digestion, but they are also the firsts to receive potentially toxic substances. Ethanol of alcoholic beverages is mainly degraded in the liver, actually in the many peroxisomes of the hepatocytes. Half of the ingested alcohol is transformed in acetaldehyde in these organelles. There are enzymes in the smooth endoplasmic reticulum involved in the degradation or inactivation of toxins and drugs. During the periods of high demand of toxic substances removal, like during medicine treatments or continuous alcohol drinking, the endoplasmic reticulum may become the most bulky organelle of the hepatocyte. Drugs are usually inactivated by conjugation with other molecules. For instances, glucosyltransferase conjugates molecules with barbiturates.

Storing and regulating iron. Hepatocytes may work for storing iron, which is concentrated in cytoplasmic depots bound to ferritin. Hepatocytes may capture iron in several ways: bound to transferrin, as part of heme groups and from non-heme groups. Transferrin-iron enter the cell by TRF1 receptor mediated endocytosis. When endocytic vesicles fuse with endosomes, transferrin releases Fe3+, which is transformed in F2+ and the extruded to the cytosol by DMT1 (divalent metal transporter 1) transporter. Heme-iron molecules are also endocyted and are translocated to the cytosol through the endosomal membrane by the HRG1 transporter. However, the majority of the iron enters from the extracellular space through the ZIP14 transporter placed in the hepatocyte plasma membrane facing the sinusoids. Once in the cytosol, the iron is bound to ferritin and stored in the cytoplasm because the free iron is toxic. Releasing the iron from the enterocyte is mediated by the ferroportin transporter found in the plasma membrane near the sinousoids.

After the bone marrow, the liver is the second major production center of heme groups. Heme group is a prostetic group (non peptidic) present in several proteins for transporting oxygen, in those enzymes like catalases and peroxidases that protect against oxidant substances. It is also part of the mitochondrial and peroxysomal cytochromes. The higher amount of heme groups is found in the hemoglobin, which are synthesized in the bone marrow. In the liver, heme group synthesis depends on the amount of microsomal p450 cytochrome needed by the cell, so that most of these heme groups are part of p450 cytochromes.

Hepatocytes release the hepcidin hormone, which regulates the systemic iron concentration in the body. This hormone controls the amount of iron in the plasma by favoring the internalization and degradation of ferroportin, iron transporter found in enterocytes, macrophages and hapatocytes. Removing ferroportin inhibits the release of iron from these cells. Hepcidin synthesis is regulated by the transferrin-iron concentration in the plasma, by iron depots in the hepatocytes and by inflammation. The erythropoyetic activity inhibit the release of hepcidin.

Bibliography

Gissen P, Arias IM. 2015. Structural and functional hepatocyte polarity and liver disease. Journal of hepatholoty. 63: 1023-1037. Read the article

Knutson MD. 2014. Adult intestinal stem cells: critical drivers of epithelial homeostasis and regeneration. Nature review in molecular cell biology. 15:19-33.

Weiss L, Greep RO. 1982. Histología. 4ªedición. Editorial el Ateneo. Barcelona.

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