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The liver is the principal metabolic organ of the body and the largest gland in the body (Cormack, 1950). It is usually included with the organs of digestion but it is much more having a variety of functions. It is essential to life and can regenerate if not totally destroyed. As much as one half of the organ will regenerate in children and it will do so according to the architectural plan of the liver if the reticular framework of the organ is preserved.
The liver is composed of hepatocytes (liver cells) that form the parenchyma or working part of the organ. The cells resemble epithelial cells: they are fairly large, close together contact one another and face a free surface. For this reason, the liver is an epithelial organ. But unlike the epithelial tissues, it has its own blood supply.
The position of the liver in the circulatory system is critical for collecting, altering, and accumulating nutrients, metabolites and toxic and foreign materials. The blood supply to the liver is unusual in that it is supplied by an artery and a vein. The hepatic artery supplies about 30% of the blood that reaches the organ. The portal vein drains the lower intestinal tract, spleen and pancreas via superior and inferior mesenteric veins and delivers 70% of the blood that reaches the liver. It contains nutrients, bile salts and much of what is absorbed by the gastrointestinal tract.
The capillary bed of the liver consists of sinusoids. The sinusoids are lined by a discontinuous layer of endothelial cells and macrophages called Kupffer cells. There is no basal lamina. The morphology permits intimate contact between the blood and liver cells and a more porous passageway for substances secreted or taken up by the liver.
The sinusoids are continuous with a series of veins called central, sublobular and heapatic veins which drain in that order into the inferior vena cava.
The liver also contains a series of ducts that drain bile from the liver. The duct system includes bile canaliculi that are formed by the membranes of adjacent liver cells, bile ducts and hepatic ducts. Through this system of conduits, bile reaches the duodenum but if the opening to the duodenum is closed the bile backs up and is stored in the gall bladder. |
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Term
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Definition
The liver is the principal metabolic organ of the body and the largest gland in the body (Cormack, 1950). It is usually included with the organs of digestion but it is much more having a variety of functions. It is essential to life and can regenerate if not totally destroyed. As much as one half of the organ will regenerate in children and it will do so according to the architectural plan of the liver if the reticular framework of the organ is preserved.
The liver is composed of hepatocytes (liver cells) that form the parenchyma or working part of the organ. The cells resemble epithelial cells: they are fairly large, close together contact one another and face a free surface. For this reason, the liver is an epithelial organ. But unlike the epithelial tissues, it has its own blood supply.
The position of the liver in the circulatory system is critical for collecting, altering, and accumulating nutrients, metabolites and toxic and foreign materials. The blood supply to the liver is unusual in that it is supplied by an artery and a vein. The hepatic artery supplies about 30% of the blood that reaches the organ. The portal vein drains the lower intestinal tract, spleen and pancreas via superior and inferior mesenteric veins and delivers 70% of the blood that reaches the liver. It contains nutrients, bile salts and much of what is absorbed by the gastrointestinal tract.
The capillary bed of the liver consists of sinusoids. The sinusoids are lined by a discontinuous layer of endothelial cells and macrophages called Kupffer cells. There is no basal lamina. The morphology permits intimate contact between the blood and liver cells and a more porous passageway for substances secreted or taken up by the liver.
The sinusoids are continuous with a series of veins called central, sublobular and heapatic veins which drain in that order into the inferior vena cava.
The liver also contains a series of ducts that drain bile from the liver. The duct system includes bile canaliculi that are formed by the membranes of adjacent liver cells, bile ducts and hepatic ducts. Through this system of conduits, bile reaches the duodenum but if the opening to the duodenum is closed the bile backs up and is stored in the gall bladder. |
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| CHARCATERISTICS OF HEPATOCYTES |
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The morphology of a hepatocyte when viewed with routine H & E preparations is rather consistent throughout the organ even though the liver is diverse in its function. Though this interpretation of the cytology may not be entirely correct (see the functional lobule below), the entire population of cells is treated as if the organ was composed of a single type of hepatocyte.
From the more simplistic standpoint of a “typical hepatocyte”, the cell contains a high concentration of many different structures. These include: SER, RER, Golgi apparatus, mitochondria, lipid droplets, glycogen and lysosomes. At the LM level, the cells are large and multisided with acidophilic cytoplasm and one or two nuclei.
Cells with two nuclei or a large nucleus are cells formed through regeneration of the hepatocytes. |
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| Macrophages (Kupffer cells) have long arms (cytoplasm processes) that extend into a sinusoid. In some instances, the part of the cell that contains the nucleus may also be seen in the sinusoid. The cells remove dead liver cells and foreign materials in the blood. In effect the processes help trap foreign materials that enter the liver through the blood. |
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| Macrophages (Kupffer cells) have long arms (cytoplasm processes) that extend into a sinusoid. In some instances, the part of the cell that contains the nucleus may also be seen in the sinusoid. The cells remove dead liver cells and foreign materials in the blood. In effect the processes help trap foreign materials that enter the liver through the blood. |
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A classical liver lobule is based on the microscopic anatomy of the tissue. It consists of a central vein in the middle of a lobule, cords of cells and sinusoids that extend from the central vein to connective tissue in the peripheral of a lobule and portal areas of connective tissue that lie that the corners of the lobule.
The hepatocytes are arranged into branching lines of cells called cords (they look like a bicycle chain). The cords are separated by clear spaces – the sinusoids - that parallel the cords. And the cords and spaces radiate away from a central cavity (central vein) something like the spokes of a wheel that radiate out from the hub of a wheel to the rim. |
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A classical liver lobule is based on the microscopic anatomy of the tissue. It consists of a central vein in the middle of a lobule, cords of cells and sinusoids that extend from the central vein to connective tissue in the peripheral of a lobule and portal areas of connective tissue that lie that the corners of the lobule.
The hepatocytes are arranged into branching lines of cells called cords (they look like a bicycle chain). The cords are separated by clear spaces – the sinusoids - that parallel the cords. And the cords and spaces radiate away from a central cavity (central vein) something like the spokes of a wheel that radiate out from the hub of a wheel to the rim. |
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| OTHER STRUCTURES IN LIVER: |
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| a space between hepatocytes and sinusoids. It forms a channel that drains lymph from the organ to the true lymphatic vessels in the portal areas. |
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| a space between hepatocytes and sinusoids. It forms a channel that drains lymph from the organ to the true lymphatic vessels in the portal areas. |
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| a channel for bile formed by adjacent hepatocytes. It is about a wide as a mitochondrion. The cavity is sealed by tight junctions that prevent bile from spilling into the blood. NOTE: the role of the tight junction is reiterated over and over; its specific function changes with the cell type and organ in which it is found. |
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| contain a bile duct lined by simple cuboidal epithelium, a branch of the hepatic artery, and a branch of the portal vein. The three structures form a triad. A lymphatic vessel is also present but not included in a triad because it is not often seen. |
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| portal areas surrounded by a plate of hepatocytes with high regenerative capacity. |
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| a space that separates the limiting plate from the portal area. |
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| a space that separates the limiting plate from the portal area. |
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| FUNCTIONAL LOBULE or LIVER ACINUS |
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The functional lobule (of Rapport) or liver acinus includes portions of two or more adjacent classical liver lobules. It is centered about the portal tract and lies between two or more central veins. The lobule is based on the flow of blood from the branches of the hepatic artery and portal vein to the central vein.
A lobule is divided into three zones. The richest blood supply first reaches ZONE 1. The cells in ZONE 1 are the first to come into contact with glucose, for example, and the first to release glucose when the level of glucose in the blood falls. The cells in ZONE 3 receive lower levels of oxygen and are the first to be damaged when the blood supply is disrupted. The cells in ZONE 1 are the last to die and the first to regenerate.
The structure of the cells in zone 1 is different than the structure of cells in zone 2 which is different from the cells in zone 3. The differences are subtle. |
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The gall bladder sits in a groove on the liver so that one side faces the peritoneal cavity and the other side is attached to the liver. The wall of the gall bladder consists of:
simple columnar epithelium that is absorptive; 2. lamina propria of loose connective tissue; 3. a muscularis of smooth muscle that forces bile into the cystic duct; and 4. serosa or adventitia depending on whether the wall faces the liver or peritoneal cavity. |
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The gland is divided by connective tissue septae into lobules but there are no radiating chains of cells. There are hues of red and blue when stained with H&E rather than the pink, orange or red of the liver and the islets of Langerhans (that do not stain well and secrete glucagon and insulin).
If anything, the pancreas resembles the parotid gland. |
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The gland is divided by connective tissue septae into lobules but there are no radiating chains of cells. There are hues of red and blue when stained with H&E rather than the pink, orange or red of the liver and the islets of Langerhans (that do not stain well and secrete glucagon and insulin).
If anything, the pancreas resembles the parotid gland. |
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