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RELATIVE DOSE 



Figure 7-6. Mutation frequency induced by EMS and BrdU 



as a function of dose. 



the plastic substratum and assume the standard epitheUal 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, CrOo and PbAc2'3H20. These were detected as v^ld-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-Kl cell, they ceased to divide and assumed a state 

 of contact inhibition, or a state resembhng that of contact inhibition. Dense 

 monolayers remained for as long as two weeks v^thout 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 



