Hepatic lipocytes, also known as Ito cells, are the principal collagen-producing cells in normal rat liver. This study isolated and cultured these cells from normal rat liver and demonstrated that they secrete collagen types I, III, and IV, as well as laminin. The amount of collagen secreted by lipocytes was significantly higher than that secreted by hepatocytes and sinusoidal endothelial cells. Lipocytes produced 10 times more collagen than hepatocytes and over 20 times more than endothelial cells, based on the production of peptide-bound [³H] hydroxyproline. The findings suggest that collagen synthesis and secretion are specialized functions of hepatic lipocytes, and that in normal liver, this represents primarily the production of type I collagen. The phenotypic resemblance of these cells to fibroblasts supports the hypothesis that lipocytes are a major source of collagen in pathologic fibrosis and may be precursors of the fibroblast-like cells observed in liver injury. The study also showed that lipocytes, when cultured, exhibit morphologic features similar to those in vivo, including the presence of vitamin A-containing cytoplasmic droplets and prominent rough endoplasmic reticulum. The collagen production by lipocytes was stable for at least 7 days in primary culture. The results indicate that lipocytes are the predominant collagen-producing cells in normal rat liver. The methods used for isolating lipocytes involved enzymatic digestion and gradient centrifugation with arabinogalactan. The study also compared collagen synthesis by lipocytes to that of hepatocytes and sinusoidal endothelial cells, and found that lipocytes produced significantly more collagen. The findings suggest that lipocytes may play a critical role in maintaining or altering the extracellular matrix in the subendothelial space.Hepatic lipocytes, also known as Ito cells, are the principal collagen-producing cells in normal rat liver. This study isolated and cultured these cells from normal rat liver and demonstrated that they secrete collagen types I, III, and IV, as well as laminin. The amount of collagen secreted by lipocytes was significantly higher than that secreted by hepatocytes and sinusoidal endothelial cells. Lipocytes produced 10 times more collagen than hepatocytes and over 20 times more than endothelial cells, based on the production of peptide-bound [³H] hydroxyproline. The findings suggest that collagen synthesis and secretion are specialized functions of hepatic lipocytes, and that in normal liver, this represents primarily the production of type I collagen. The phenotypic resemblance of these cells to fibroblasts supports the hypothesis that lipocytes are a major source of collagen in pathologic fibrosis and may be precursors of the fibroblast-like cells observed in liver injury. The study also showed that lipocytes, when cultured, exhibit morphologic features similar to those in vivo, including the presence of vitamin A-containing cytoplasmic droplets and prominent rough endoplasmic reticulum. The collagen production by lipocytes was stable for at least 7 days in primary culture. The results indicate that lipocytes are the predominant collagen-producing cells in normal rat liver. The methods used for isolating lipocytes involved enzymatic digestion and gradient centrifugation with arabinogalactan. The study also compared collagen synthesis by lipocytes to that of hepatocytes and sinusoidal endothelial cells, and found that lipocytes produced significantly more collagen. The findings suggest that lipocytes may play a critical role in maintaining or altering the extracellular matrix in the subendothelial space.