Development of intralobular bile ductules after spontaneous hepatitis in Long-Evans mutant rats.

Oval cell proliferation occurs during spontaneous hepatitis in Long-Evans cinnamon (LEC) rats. It has been reported that oval cells undergo differentiation into mature hepatocytes via small hepatocytes during carcinogenesis. This study was designed to demonstrate in vivo differentiation of oval cells into typical bile ductular cells in the liver lobule and the characteristic feature of intralobular bile ductule formation in LEC rats. We have examined kinetics, intralobular distribution, and morphology of oval cells, small hepatocytes, and bile ductular cells in LEC rat livers at prehepatitic, acute hepatitic, chronic hepatitic, and precancerous stages by conventional light and electron microscopy, immunostaining for cytokeratin, and 3-dimensional reconstruction analysis. Our results indicate that oval cells proliferated and extended into the periportal zone of the liver lobule during acute hepatitis at 20 to 23 weeks after birth. They exhibited tubular structures with a poorly defined lumen and incomplete basement membrane. After remission of the jaundice, small hepatocytes proliferated in association with oval cells and predominated in the periportal zone at 26 weeks. In a chronic hepatitic stage at 28 to 30 weeks, tubular structures were transformed into typical bile ductules, which had a well defined lumen and complete basement membrane, and small hepatocytes became a normal size. Intralobular bile ductules originated from the interlobular bile ducts, ran in the space of Disse, giving rise to several branches in the course, and were terminated at the hepatocytes. The present results indicate that oval cells that proliferate in the liver lobule of LEC rats after spontaneous hepatitis not only differentiate into small hepatocytes but also into typical bile ductular cells. This study suggests that intralobular bile ductules may play roles in maintaining the bile excretion during and after the disorganized proliferation of oval cells and small hepatocytes.