Immunolocalization of extracellular matrix components and integrins during mouse liver development

Intrahepatic biliary cell differentiation takes place in periportal hepatoblasts under the influence of the subjacent connective tissue, the mechanism of which is still unclear. This study was undertaken to analyze the immunolocalization of extracellular matrix components and their cellular receptors during mouse liver development, with special attention given to biliary differentiation and vascular development. In young fetal mouse liver, primitive structures of sinusoids were developed between hepatic cords associated with hematopoietic cells demonstrated by immunohistochemistry of basal laminar components, the alpha6 integrin subunit, and PECAM-1. Portal veins and hepatic veins showed different staining intensities of alpha2, alpha3, and alpha6 integrin subunits from early stages of development. Anti-beta4 integrin subunit antibodies reacted with portal veins, but not with hepatic veins after perinatal stages. Their different phenotypes may be related to the preferential differentiation of periportal bile ducts. In intrahepatic bile duct development, periportal hepatoblasts adjacent to the connective tissue were immunostained for each basal laminar component on the basal side at almost the same time; alpha3, alpha5, alpha6, and beta4 integrin subunits were immunohistochemically detectable later than the basal laminar components. These staining patterns of intrahepatic bile duct cells clearly differed from those of extrahepatic bile duct cells from the beginning of their development, suggesting that these ducts are of different origins. In conclusion, the vascular structures, including sinusoids, portal veins, and hepatic veins, develop from early stages of liver development, and the extracellular matrix components may play important roles in biliary differentiation and vascular development.