Control of Nuclear Division in Normal but Not in Neoplastic Mouse Cells1

A control mechanism that limits nuclear division is pres ent in contact-inhibited nontumorigenic mouse BALB/3T3 cells and mouse embryo fibroblasts. In the absence ofcytoplasmic division, 3T3 and mouse embryo fibroblast cells may undergo nuclear division once or occasionally twice and then stop. Also, there is no increase in the frequency of premature chromosome condensation. In non-contactinhibited tumorigenic BALB/3T12 and L-cells, nuclear division occurs repeatedly in the absence of cytoplasmic division and is therefore relatively uncontrolled. The fre quency of premature chromosome condensation is mark edly increased in 3T12 and L-cells unable to undergo cyto plasmic division. These observations are consistent with the hypothesis that the loss of control of nuclear division in neoplastic cells represents a characteristic defect.