Murine embryonal carcinoma hybrids: decreased ability to spontaneously differentiate as a dominant trait

The F9 murine embryonal carcinoma (ec) cell line does not differentiate spontaneously at high frequency either in vitro or in vivo. A derivative of this variant ec cell line resistant to ouabain and thioguanine was fused with the PSA1 ec cell line, which retains the ability to differentiate to a variety of cell types, either in vitro or in vivo. Hybrids were selected in media containing ouabain, hypoxanthine, aminopterin, and thymidine at a frequency of approximately 10(-3). Authentic hybrids were chosen by screening for the expected hybrid DNA content by flow microfluorometry and the lack of parental cell contamination in appropriate selective media. After injection into strain 129 mice, five or eight independently isolated hybrids formed tumors composed exclusively of embryonal carcinoma cells. The remaining three hybrids of this cross caused tumors composed of greater than 95% embryonal carcinoma cells but with small foci of differentiated tissues as well. Like the F9-derived parental line, less than 10 percent of the clones, formed by the hybrids after six days of growth, contained cells that secreted plasminogen activator. Since control hybrids derived from the fusion of either two F9 derivatives or two pluripotent cell lines resembled their parental cell lines with regard to the types of tumors formed and their differentiation in vitro, it appears that the F9 phenotype is dominant to the pluripotent phenotype in early passage embryonal carcinoma hybrids.