Relationship of cell growth behavior in vitro to tumorigenicity in athymic nude mice

The serum requirements, anchorage requirements, saturation densities, and contact inhibition responses of a variety of mammalian cell lines were determined under uniform conditions. The serum requirement of both transformed and normal cells was a sensitive function of initial plating density. Cloning efficiency on irradiated mouse monolayers was found to be an invalid indicator of contact inhibition of growth, since most cell lines that failed to form visible colonies on cell monolayers nonetheless proliferated on these monolayers. When normal and neoplastic cells from a variety of sources were examined, none of the growth parameters that serve to define the transformed state in vitro correlated consistently with cellular tumorigenicity in athymic nude mice. It is concluded that the most reliable and physiologically meaningful assay for malignant transformation is, at present, cellular tumorigenicity in athymic nude mice.