[The differentiation of oval cells into hepatocytes during induced hepatic carcinogenesis in mice. An electron microscopic study].

An electron microscopic study of murine oval cells, induced by a single injection of genotoxic agent dipin and by a partial hepatectomy, has shown that their ultrastructure and direction of differentiation depend on localization in the liver lobule. Oval cells around portal tracts go through three stages of development: low differentiated cells 4.40 +/- 0.51 mu in diameter with ovoid nuclei 3.43 +/- 0.44 mu, intermediate cells, and young hepatocytes. They form common ducts surrounded by a basal lamina, and produce bile canaliculi-like structures and intermediate junctions between them. Another part of the oval cell population is organized similar to the bile duct epithelium. It consists of cells 9.37 +/- 1.1 mu in diameter with nuclei 7.28 +/- 1.16 mu in diameter and form a system of branching and anastomosing ducts widespread along the parenchyma from the portal to the central veins. Our data indicate that the oval cells can differentiate into hepatocytes, and support a hypothesis according to which the cells of terminal bile ductules are liver epithelial stem cells which can differentiate into a hepatocyte or a bile duct cell lineage in periportal microenvironment.