Ultrastructural localization of type IV collagen, laminin and prolyl hydroxylase in biliary epithelial cells of rat liver following ligation of the common bile duct

Abstract

Type IV collagen and laminin are major components of basement membrane (BM), whereas prolyl hydroxylase (PH) is a key enzyme in the hydroxylation of proline to hydroxyproline in collagen synthesis. In order to elucidate the exact mechanism of the formation of BM, immune electron microscopic observation of type IV collagen, laminin and PH was made in rat liver with marked proliferation of bile ducts following ligation of the common bile duct. Extracellular localization of type IV collagen was found in the BM of bile ducts and blood vessels and in the space of Disse in both normal rat liver and the liver of rats undergoing operation. Type IV collagen was localized in lamina rara and lamina densa. Laminin was codistributed with type IV collagen in BM but rarely in the space of Disse even in the liver of rats undergoing operation. Immunostaining of laminin was diffusely spread in lamina densa, but sparsely in lamina rara. Though no reaction products of type IV collagen and laminin were detected in the cytoplasm of normal biliary epithelial cells, they were found in rough endoplasmic reticulum (rER) and the vesicles close to the basal surfaces of the plasma membrane of the proliferating biliary epithelial cells. No evident localization of these components in Golgi apparatus was found. PH was found in rER of the biliary epithelial cells, hepatocytes, endothelial cells of vessels, fibroblasts and perisinusoidal cells except for Kupffer cells in normal rat liver. More intense and diffuse staining of PH was observed in rER in the proliferating biliary epithelial cells of the liver of rats undergoing operation in concomitance with the evident localization of type IV collagen in this organelle. These findings suggest that the major components of BM, such as type IV collagen and laminin in the proliferating biliary epithelial cells, are produced in rER and secreted by vesicles to the basal extracellular spaces, thus forming new BM in these circumstances.