Rapid induction of foci escaping density-dependent inhibition in baby mouse skin cultures.

Mesenchymal cells from primary BMS (baby mouse skin) cultures formed secondary monolayers subject to density-dependent inhibition. The monolayers remained quiescent but in good condition for 6-8 weeks if given weekly medium changes. Exposure of the primary cultures to fluorescent light and/or oxygen produced "altered" cells that gave rise in the secondary cultures to foci of up to 10(4) cells after 15 days. These foci overgrew the background BMS cells. The rate of growth, morphology, and arrangement of the altered cells varied greatly between foci but much less within a focus, which usually showed one or more characters of neoplastic cells. The initiation of foci was apparently not transmissible by an infectious agent.