Oval cells--hepatocytes relationships in Dipin-induced hepatocarcinogenesis in mice.

During Dipin-induced hepatocarcinogenesis in mice there is powerful and prolonged proliferation of oval cells which are arranged in duct-like structures. Ultrastructure and differentiation pathways of oval cells depend on their location in the liver lobule. The major part of oval cells is represented by duct lining cells morphologically similar to biliary epithelial cells. They form the system of branching anastomozing ducts and expand into the parenchyma from portal to central veins. Later these ducts disintegrate. In the periportal areas, three stages of oval cell differentiation can be distinguished: (1) low differentiated cell similar to cells of terminal biliary ductules in their size and ultrastructure, (2) transitional cells and (3) young hepatocytes. Cells with ultrastructural characteristics of sequential stages of hepatocyte differentiation are located within the ducts surrounded by the basal lamina. Our data suggest that oval cells are the committed cell precursors capable of differentiating into hepatocytes or biliary epithelial cells in the periportal microenvironment.

[The stem-cell reserve of the liver]. / Stvolovoi rezerv pecheni.

This paper considers the problem of epithelial stem cells of the liver and possibilities of its experimental solution. Authors' own data obtained with the model of induced hepatocarcinogenesis in mice are discussed; the experiments were performed using electron microscopy, autoradiography and immunochemistry. In accordance with these data, Gering cells are stem cells of the liver, and oval cells correspond to committed precursors capable to differentiation in either hepatocellular or cholangiolar direction under the conditions of periportal microenvironment. We have also compared hepatocyte differentiation in preneoplastic mouse liver and regenerating pancreas of adult rats (Rao et al. Amer. J. Pathol. 1989. V. 134. P. 1069-1086). We also discuss stem cell compartment organization in organs having glandular structure and the possibility of existence in the adult of non-committed multipotent cells capable of producing various types of differentiation in tissues having common origin during embryogenesis.

[The formation of oval-cell ducts during hepatic carcinogenesis in mice. Its relationship to the pre-existing canals of Hering]. / Obrazovanie protokov oval'nykh kletok v protsesse gepatokantserogeneza u myshei. Sviaz' s predsushchestvuiushchimi kanalami Geringa.

It has been shown that a population of the oval cells is formed in mouse liver during the dipin-induced carcinogenesis (Radaeva, Factor, 1990b). This paper deals with the origin of the oval cells and their proliferation potential depending on localization in the liver lobule. Series of semithin liver sections were studied under the light microscope and detected labeled cells analyzed under electron microscope on serial ultrathin sections. We found that proliferation of cells of terminal bile ductules (Hering [correction of Gering] canals) takes place at the early stages of liver carcinogenesis. These cells and first labeled oval cells had similar size and morphology and jointly formed the ducts. Oval cell population was heterogeneous in terms of proliferative potential. Proportion of proliferating cells (38-45%) in the oval cells of Hering [correction of Gering] canals and small ducts surrounding portal tracts remained similar throughout the period of formation of the oval cell population. In the oval cells infiltrating the parenchyma, the proportion of proliferating cells appeared to depend on the intensity of the oval cell response: it attained the maximum (62%) on intermediate stage and decreased to the minimum (22%) at the peak of the reaction. These data suggest that Hering [correction of Gering] canals probably give origin to the ducts formed by oval cells.

Origin and fate of oval cells in dipin-induced hepatocarcinogenesis in the mouse.

We have studied the development and differentiation of oval cells in the Dipin model of hepatocarcinogenesis in the mouse and compared this process to generation of biliary epithelial cells by bile duct ligation using light and electron microscopy. The Dipin model of hepatocarcinogenesis consists of a single injection of an alkylating drug, Dipin (1,4-bis[N,N'-di(ethylene)-phosphamide]-piperazine), followed by partial hepatectomy. The Dipin treatment resulted in irreversible damage and gradual death of hepatocytes by necrosis and apoptosis. Earlier work provided evidence that regeneration of parenchyma occurred via oval cell proliferation and subsequent differentiation into hepatocytes that replaced the degenerating hepatocytes. Both autoradiographic and morphological data indicated that oval cells were derived from ductular cells of Hering canals. The first oval cells labeled with [3H]thymidine were similar in size and ultrastructure to ductular cells of Hering canals with whom intracellular connections existed. The proliferation of ductular cells of Hering canals gave rise to a new system of oval cell ducts that spread into the liver acinus. In the periportal areas, the transition of oval cells into hepatocytes was observed inside the ducts. Both growth patterns and ultrastructure of oval cells were different from the biliary epithelial cells in bile duct-ligated liver. Also, oval cells retained the property to interact with adjacent hepatocytes through desmosomes and intermediate junctions. Oval cell population was heterogeneous in terms of proliferating potential. A proportion of proliferating cells (38 to 45%) in the Hering canals and small oval cell ducts located in the periportal areas was similar throughout the period of oval cell development. The extent of proliferation of oval cells decreased from 62% at the stage of active migration into the acinus to 22% at maximum formation of oval cell ducts. These data suggest that in the mouse liver cells of the terminal biliary ductules harbor the hepatic stem cell compartment from which oval cells, capable of differentiating into hepatocytes, may be derived.