Immunocytochemistry of perinatal rat livers with a special reference to the roles of mesenchymal cells in hepatic differentiation.

To investigate the roles of extracellular matrix produced by hepatic mesenchymal cells in the organization of hepatic cell cords, perinatal rat livers were examined with immunocytochemistry of fibronectin (FN) and laminin (LM). Some hepatocytes in a free state at prenatal day 15 actively produced FN and LM in the rough endoplasmic reticulum but lost this synthetic activity when such cells were incorporated into hepatic cell cords. On the other hand, hepatic mesenchymal cells, especially those associated with the perisinusoidal space, retained this synthetic activity throughout the stages examined. In the differentiating hepatic cell cords, positive immunoreactions for FN and LM were preferentially seen on the cell surface facing both sinusoidal space and differentiating bile canaliculus concomitant with the expression of the tight junction protein, ZO-1, from prenatal day 17. Since such hepatocytes have lost or reduced their synthetic activities of both glycoproteins in the rER, the immunoreactions appear to be mainly due to hepatic mesenchymal cells which seem to play a role in the formation of the hepatic cell cords and the bile canaliculi.