lOCh 
RELATIVE DOSE 
Figure 7-6. Mutation frequency induced by EMS and BrdU 
as a function of dose. 
the plastic substratum and assume the standard epithelial morphology (Figure 
7-7). Although this type of cell produces colonies of sufficient size to permit 
cloning, the cells are continuously in a rounded state, as if entering mitosis. 
When grown in medium containing 10 percent fetal calf serum, some of the 
clones assume a more normal morphology, and may represent mutants with 
increased serum requirements (27). To date, this type of variant has been 
observed only in populations treated with EMS, BrdU, and compounds of 
chromium, cadmium and lead. 
A second type of variant, appearing to possess the property of contact 
inhibition (1, 19), was isolated from cell populations treated with the known 
carcinogens, EMS, CrO^ and PbAc2*3H-»0. These were detected as wild-type 
cells surviving selection, and which possessed a pronounced fibroblastic 
morphology (Figure 7-7). When cells were grown into confluent monolayers, 
unlike the transformed CHO-K1 cell, they ceased to divide and assumed a state 
of contact inhibition, or a state resembling that of contact inhibition. Dense 
monolayers remained for as long as two weeks without medium changes, and 
without significant deterioration. Confluent sheets of cells could be easily 
trypsinized and dispersed into uniform, single-cell populations. Upon replating 
in fresh growth medium, cells grew with a generation time of approximately 14 
hours, ceasing to divide when the monolayer again became confluent. 
90 
