Biliary Epithelial and Hepatocytic Cell Lineage Relationships in Embryonic Rat Liver as Determined by the Differential Expression of Cytokeratins, α-Fetoprotein, Albumin, and Cell Surface-exposed Components

Abstract

The differentiation patterns of epithelial cells in fetal rat liver were analyzed in situ and in primary culture by indirect immunofluorescence microscopy using polyvalent and monoclonal antibodies directed against cytokeratins with molecular weights of 55,000 (CK55), 52,000 (CK52), and 39,000 (CK39) and against vimentin, albumin, α-fetoprotein, and surface-exposed components of bile ductular cells (BDS₇) and hepatocytes (HES₆). The anti-CK52 antibody, which reacted with biliary ductal cells in the liver of adult rats (Germain et al., Cancer Res., 45: 673, 1985; Germain et al., Cancer Res., 48: 368–378, 1988), stained essentially all of the epithelial cells of embryonic day 12 (E12) rat liver. The anti-BDS₇ antibody reacted with a few cell foci, which enlarged and became more numerous at later developmental ages. At E12 essentially all of the cells were positive for albumin and α-fetoprotein but did not express HES₆. In fact HES₆ was not detected until E15 in cells with the morphology of immature hepatocytes. By E18 staining with anti-HES₆ reached the level of that observed on adult rat hepatocytes. Liver cells isolated from E12 rats were seeded on fibronectin-treated dishes and their response to various combinations of growth- and differentiation-promoting factors was evaluated with respect to their capacity to express either the hepatocytic or the bile ductular phenotype. In medium supplemented with serum, insulin, dexamethasone, and dimethyl sulfoxide, the E12 cells were capable of differentiating in culture to mimic over a 6-day period the sequential phenotypic changes which occur in vivo during normal hepatoontogeny, namely the loss of CK52 and the appearance of HES₆. In contrast, the addition of sodium butyrate to the above supplement mixture resulted in the massive expression of BDS₇. To further assess the developmental potential of fetal rat liver cells toward the biliary epithelial cell lineage, the in vitro assay was performed using cells isolated from livers of E18 rats and also from 2-day-old (P2) and P14 rats. While a slight expression of BDS₇ was induced in cell culture from E18 liver, essentially no expression was observed in cells from postnatal livers. These findings strongly suggest that the emerging hepatic tissue in rat embryo is composed of bipotential progenitor epithelial cells that are capable of differentiating along either the hepatocytic or biliary epithelial cell lineage. These observations constitute a clear demonstration of the plasticity of liver differentiation and also provide a striking example of environmental influences on liver progenitor cell differentiation.