Ultrastructure of invasion of the adult rat hepatocyte monolayer by rat ascites hepatoma variant sublines.

Invasion of a primary cultured adult rat hepatocyte monolayer by rat ascites hepatoma AH7974-derived variant sublines (74AD, 74FL) was studied ultrastructurally. Although 74AD and 74FL are an adherent and a floating subline, respectively, in ordinary in vitro culture, the majority of the cells of both sublines attached to the hepatocytes at the tips of their cytoplasmic processes up to 2 h after coculture. Thereafter, these cells adhered to the hepatocytes with several types of junctional structures. Simple apposition was prominent 6 h after coculture, and intermediate and tight junctions were frequently observed 12 h after coculture. The attached tumor cells separated the intercellular junctions of the hepatocyte monolayer with their cytoplasmic protrusions and became directly adherent to dishes by pushing aside the hepatocytes. In addition, we sometimes found some peculiar interactions which appeared to be an early event of a kind of cytoplasmic fusion between a tumor cell and a hepatocyte after 12 h of coculture. An increasing electron density of mitochondria, the disappearance of cristae, and the presence of cytoplasmic vacuolations were observed in the hepatocytes fused with tumor cells. Such results suggest that the hepatic invasion by tumor cells results from the interaction of tumor cells and hepatocytes leading to their cytoplasmic fusion, as well as to direct migration of tumor cells into hepatic cords. The mechanism of invasion may be independent of that of substrate adhesion.