Partial hepatectomy or carbon tetrachloride (CCl4) injury, following treatment of rats with 2-acetylaminofluorene (2-AAF) to inhibit proliferation of hepatocytes, induces proliferation of oval cells and possibly their differentiation into nodular foci of hepatocytes when higher doses of 2-AAF are used. Unfortunately, immunohistochemistry in previous studies failed to show oval cell markers in these foci, and thereby to demonstrate the precursor-product relationship between oval cells and hepatocytes. Immunohistochemistry on livers of rats treated with high dose 2-AAF/CCl4 was used. We found 7.6% of the hepatocyte foci were positive for an oval cell marker cytokeratin 19 (CK-19). These foci were positive for alpha-fetoprotein, less positive for carbamoylphosphate synthetase 1, and more positive for laminin in the basement membrane lining. Rarely present transitional foci had weaker expression of CK-19 and discontinuous laminin. Focal hepatocyte differentiation of oval cells was characterized by cell hypertrophy, membranous CK-19, and positive hepatocyte nuclear factor 4 (HNF-4). HNF-4(+) small oval cells surrounding CK-19(+) foci were frequently seen, suggesting that a paracrine mechanism(s) may be responsible for the enlargement of CK-19(+) foci. In conclusions, oval cells appear to differentiate to CK-19(+) foci and then to CK-19(-) foci in the high dose 2-AAF/CCl4 model.
