PROTEIN SYNTHESIS AND TURNOVER IN HUMAN CELL CULTURES 7OI 
A number of similar examples of a population-dependent requirement have now 
been observed. Thus, all but one of the mammalian cell cultures so far examined 
had a rigorous requirement for inositol, despite the fact that when they were given 
pre-formed inositol, approx. 25° of the total inositol residues of the cell were being 
synthesized from the glucose of the medium!’. At a population density of approx. 
200 000 HeLa cells/ml, the inositol became unnecessary. The cells grew in the absence 
of added inositol, provided only that the population density was maintained in 
excess of 200 000 cells/ml. 
Another example of this phenomenon was a mouse leukemia cell cultivated by 
HERZENBERG AND Roosa™, which required either serine or pyruvate for growth. 
This requirement disappeared if the population density of this cell line was main- 
tained in excess of approx. 50 000-150 000 cells/ml. 
Glutamine provides yet another example of population-dependent requirements 

Fig. 4 (b). The profound structural alterations produced in the HeLa cell by valine deficiency 
(from COHEN, NYLEN AND Scott’), reversible on the restoration of valine to the medium. 
References p. 705 
