REGULATING FACTORS OF THE PFS 



523 



(3) The amount of labelled amino acids present in the protein and in the pool of 

 free amino acids in the cell must be measured over a time span which includes both 

 the increasing and decreasing phase of specific activity. 



Applying these prerequisites the incorporation of radioactive glycine into the 

 glycine moiety of actomyosin and collagen, obtained from the leg musculature of 

 the 14-day chick embryo and the 7-day chick indicates, as discussed previously, 

 (p. 501) that at least a large part of the increase in the specific activity in the 

 proteins is a measure of actual net synthesis (Herrmann, Lerman and White, 



1958). 



For a meaningful interpretation of quantitative measurements it has to be kept 

 in mind that the formation of increasing amounts of a specific cell protein can be 

 the result of entirely different situations relative to overall protein production. The 

 appearance and the increase of new types of proteins may be concomitant with, 

 and merely an expression of, an increase in the capacity of the entire protein form- 

 ing apparatus. One might think of the possibility that the centers of protein pro- 

 duction increase in total number or that the energy supply required for protein 

 synthesis increases. Under these circumstances it would be more likely that a 

 greater number of new protein types would appear and accumulate at an increased 

 rate {e.g. the simultaneous appearance and accumulation of enzymes in the 

 mesenchyme blastula of the sea urchin). If the appearance and accumulation of 

 these new protein types represents a true overall increase in the productivity of 

 the entire PFS, this would lead to an increase in cell proliferation, in the protein 

 content per cell, or in both. 



An alternative situation would be the synthesis of specific proteins with an 

 unchanged rate of total protein production. In this case, appearance and accumu- 

 lation of new proteins would be possible only if the formation of other protein 

 types would be reduced to a correspondingly lower rate. This reduction could 

 be an expression of a reduced growth rate, i.e. reduced formation of new cells. 



The appearance of new proteins could occur possibly during a period when 

 overall synthesis is reduced either by a cessation of the proliferative growth or by 

 the decrease of protein production by the individual cell. There is, of course, no 

 reason to assume that the one or the other of these possibilities is a generally valid 

 scheme for the differentiation of all tissues of every species. To the contrary, it must 

 be expected that in the course of differentiation several of the proposed mecha- 

 nisms will be operating. 



{c) The relation of proliferation and differentiation 



It has been generally assumed that in rapidly proliferating cells differentiation 

 is suppressed. In the absence of quantitative data it seemed that during phases of 

 rapid proliferation the specific cell products were not formed and only when pro- 

 liferation was retarded or completely abolished a rapid accumulation of specific 

 cell products could occur. Complete incompatibility could be defined more 

 precisely in the development of ascidians. In these organisms Berrill (1935) 

 determined for each cell type the numbers of cell divisions which precede the first 

 cytological appearance of the differentiation products in the developing buds. 

 For example, at an early stage of development when the thoracic mesenchyme 



Literature p. sjg 



