OVAL CELL-DIFFERENTIATION INTO HEPATOCYTES IN THE ACETYLAMINOFLUORENE-TREATED REGENERATING RAT-LIVER

When hepatocyte regeneration after a two-thirds partial hepatectomy (P H) in rats is blocked by oral gavage of acetylaminofluorene, a prolife ration of ductular cells ensues that results in a profusion of neoduct ules radiating from each portal tract. To examine the possibility that this population of newly emerging cells harbors cells capable of diff erentiating into hepatocytes, we have looked in these cells for expres sion of functional markers of hepatocyte commitment at both the RNA an d protein levels, Expression of albumin and alpha-fetoprotein (alpha-F P) messenger RNA (mRNA) transcripts were sought in situ using antisens e riboprobes, and the expression of a number of cytochrome P450 enzyme s was examined immunohistochemically. Before any signs of differentiat ion the ductular cells strongly expressed cytokeratins 7, 8, 18, and 1 9 in the same manner as authentic bile ducts, but unlike the latter al so expressed vimentin. lit situ hybridization studies showed that smal l bile ducts close to the limiting plate, as well as the newly formed ducts, expressed albumin and alpha-fetoprotein messenger RNAs, and imm unocytochemistry showed that the distribution of the respective protei ns was similar. Beginning at 1 week after partial hepatectomy, areas o f differentiation could be found in the new ducts, with cells resembli ng either columnar intestinal-type epithelia or hepatocytes. Intestina l-like cells expressed neither albumin, alpha-FP, nor cytochrome P450 enzymes, whereas ductular cells appearing Like hepatocytes with the ty pical membranous distribution of cytokeratin 8 strongly expressed a va riety of cytochrome P450 enzymes normally associated with functional h epatocytes. These observations further support the belief that reactiv e ductules, sprouted from small ducts, can represent an adaptive respo nse of the Liver to replenish lost hepatocytes, although some of the n ewborn cells appear to differentiate along intestinal lines.