Vitamin stellate cell storage

Excessive intake of vitamin A (hyper-vitaminosis A), like too little intake, can result in adverse health consequences. Approximately 60%-80% of vitamin A is stored in the liver in hepatic stellate cells (also called Ito cells and fat-storing cells). Retinyl esters are the main storage form of vitamin A in the liver and are found in lipid droplets present in the hepatic... 

The retinyl esters are incorporated into chylomicrons, which in turn enter the lymph. Once in the general circula-tion. chylomicrons arc converted into chylomicron remnants, which arc cleared primarily by the liver. As the c.stcrs enter the hepalocytes. they are hydrolyzed. In the endoplasmic reticulum, the retinol is bound to retinol-binding protein (RBP). This cotnplex is released into the blood or transferred to liver stellate cells fur storage. Within the stellate cells, the retinol is bound to CRBP(I) and e.stcnTicd for storage by ARAT and LRAT. Stellate cells contain up to 95% of the liver vitamin A. stores. The RBP-retinol complex released into the general circulation from hepalocytes or stellate cells, in turn, is bound to transthyretin (TTR), which protects retinol from metabolism and renal excretion. ... 

The major functioning cells in the liver are the hepato-cytes, responsible for about 70% of liver mass and performing most of the metabolic and synthetic functions of the liver. Two other cell types are found in small numbers within the liver. The stellate cells (sometimes referred to as Ito cells) are located between the endothelial lining of sinusoids and the hepatocytes within a small cleft referred to as the space of Disse. In their normal, quiescent state, stellate cells serve as a site of storage for fat soluble vitamins, particularly vitamin A. Wlien stimulated, stellate cells are transformed both morphologically and functionally. They synthesize collagen, and are the cells responsible for fibrosis and, eventually, cirrhosis. They synthesize nitric oxide, which helps to regulate intrahepatic blood flow. 

The stellate cells are also called perisinusoidal or Ito cells. There are approximately 5 to 20 of these cells per 100 hepatocytes. The stellate cells are lipid-filled cells (the primary storage site for vitamin A). They also control the turnover of hepatic connective tissue and extracellular matrix and regulate the contractihty of the sinusoids. When cirrhosis of the liver is present, the stellate cells are stimulated by various signals to increase their synthesis of extracellular matrix material. This, in turn, diffusely infiltrates the hver, eventually interfering with the function of the hepatocytes. 

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