Abstract
NIH 3T3 cells that are passaged frequently at low density in high (10%) calf serum lose their original capacity to produce transformed foci on a monolayer of nontransformed cells. They can then be used to form a monolayered background for the assay of the number of focus-forming cells from a transformed population. Continuation of the low-density passages for many weeks gives rise to a population that can suppress the full development of foci by a transformed line. The suppression appears to occur only after the background cells have become confluent and contact inhibited. It can also cause the disappearance of light foci that had developed before suppression began. Another subline of cells that were passaged at cloning density only once a week lose their focus-forming capacity more slowly than those passaged thrice weekly. When used as a background for the assay of a transformed line, they permit continuous expansion of the foci, with no sign of suppression. Not only the number and size of foci but also their detailed morphology is influenced by the background on which they are formed. A suppressive background can also determine the spatial distribution of foci, presumably as a result of gradients in local cell density of the background. The permissiveness of a nontransformed cell population for focus formation by transformed cells appears to be related to the capacity of the nontransformed population itself to undergo transformation when exposed to the constraints used to induce transformation. These findings indicate there are many degrees of capacity to suppress focus formation and to overcome suppression. They have significance for tumor development and for the epigenetic interactions of normal development.