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Quiescent stellate cells represent 5-8% of the total number of liver cells. [4] Each cell has several long cytoplasmic protrusions that extend from the cell body and wrap around the sinusoids. [5] The lipid droplets in the cell body store vitamin A as retinyl palmitate. [6] Hepatic stellate cells store 50–80% of the body's vitamin A. [6]
The perisinusoidal space also contains hepatic stellate cells (also known as Ito cells or lipocytes), which store vitamin A in characteristic lipid droplets. [2] This space may be obliterated in liver disease, leading to decreased uptake by hepatocytes of nutrients and wastes such as bilirubin.
The capacity to store retinol in the liver means that well-nourished humans can go months on a vitamin A deficient diet without manifesting signs and symptoms of deficiency. Two liver cell types are responsible for storage and release: hepatocytes and hepatic stellate cells (HSCs).
Research has shown the pivotal role of the stellate cell, that normally stores vitamin A, in the development of cirrhosis. Damage to the liver tissue from inflammation leads to the activation of stellate cells, which increases fibrosis through the production of myofibroblasts, and obstructs hepatic blood flow. [60]
About 70–85% of the liver volume is occupied by parenchymal hepatocytes. Nonparenchymal cells constitute 40% of the total number of liver cells but only 6.5% of its volume. [27] The liver sinusoids are lined with two types of cell, sinusoidal endothelial cells, and phagocytic Kupffer cells. [28] Hepatic stellate cells are nonparenchymal cells ...
Liver cell death and inflammatory responses lead to the activation of hepatic stellate cells, which play a pivotal role in hepatic fibrosis. The extent of fibrosis varies widely. Perisinusoidal fibrosis is most common, especially in adults, and predominates in zone 3 around the terminal hepatic veins. [25]
Unlike the other twelve vitamins, vitamin D is only conditionally essential - in a preindustrial society people had adequate exposure to sunlight and the vitamin was a hormone, as the primary natural source of vitamin D was the synthesis of cholecalciferol in the lower layers of the skin's epidermis, triggered by a photochemical reaction with ...
CYP2R1 is cytochrome P450 2R1, an enzyme which is the principal vitamin D 25-hydroxylase. [5] [6] In humans it is encoded by the CYP2R1 gene located on chromosome 11p15.2. [7]It is expressed in the endoplasmic reticulum in liver, where it performs the first step in the activation of vitamin D by catalyzing the formation of 25-hydroxyvitamin D. [8]