AbstractPrimary cultures of baby mouse kidney epithelial cells can grow without fibroblast overgrowth in a hormone‐supplemented serum‐free medium (Medium K‐1) designed for an established kidney epithelial cell line, MDCK. The five supplements in Medium K‐1 are insulin, transferrin, PGE1, T3, and hydrocortisone. Medium K‐1 also supports the growth of kidney epithelial cell cultures from a number of animls, including man, withoug fibroblast overgrowth. Outgrowth of kidney epithelial cells from kidney explants was also observed with Medium K‐1. Thus, the medium appears to be selective for epithelial cell growth.The physiological properties of primary cultures of baby mouse kidney epithelial cells were studied in detail. Baby mouse kidney epithelial cells grew at equal rates (0.5 doublings/day) in Medium K‐1 and serum‐supplemented medium. Medium K‐1 also supported the formation of baby mouse kidney epithelial colonies at low cell densities. The dependence of baby mouse kidney epithelial colony formation upon the five factors in Medium K‐1 was examined. These studies indicated that the formation of baby mouse kidney epithelial colonies in defined medium depended upon all the five supplements in Medium K‐1, in a manner similar, although not identical, to MDCK colonies.Primary cultures of baby mouse kidney epithelial cells grown in Medium K‐1 retained kidney cell‐associated properties, including the ability to form multicellular domes, a phenomenon associated with transepithelial salt transport. Amiloride‐sensitive Na+uptake and the mucosal surface enzyme leucine aminopeptidase were also observed in baby mouse kidney cultures. Similar functions were
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