Morphology of factor-independent fetal liver cell lines. (A) For comparison, mast cells obtained from fetal liver cultures infected with RufNeo-hβc and cultured for 6 weeks in the presence of growth factors (IL-3, GM-CSF, and Epo). (B and C) RTVE1 and RTVE2 cell lines, respectively, obtained after infection of fetal liver cells with RufNeo-V449E. Photographs are at 780× original magnification.