THE MITOTIC CYCLE 



(differentiation). That maintenance, growth and differentiation can be 

 distinguished conceptually does not necessarily mean that they are 

 distinct in the embryo' — or, it might be added, in any tissue or in the 

 cell itself. 



Mitosis takes place in tissues in process of normal development, 

 repair and regeneration, and in tumours. It is probable that the basic 

 conditions leading to cell division are the same in all these states. So 

 the question why cell division happens is part of the larger problem 

 of why it sometimes ceases to occur. If it were understood why cell 

 division largely ceases in maturity, or when repair and regeneration 

 are achieved, it might become clearer how cells are released from this 

 control in tumours and, rather similarly in tissue cultures (Berrill^). 



Research on cell growth, especially in tissue culture, has often been 

 conducted on the assumption (stated or implied) that there is a single 

 growth substance or 'growth hormone', the addition of which to a 

 system otherwise favourable physically and chemically acts as 'the' 

 stimulus to cell division. It is not in fact known whether such a single 

 substance exists. There may be one key point at which the metabolism 

 of the cell may regularly be altered, by the application of one special 

 stimulus (or perhaps one of several), in the direction leading inevitably 

 to mitosis. Berrill^ is one of those who do not regard cell division as 

 the result of a single, simple stimulus. On the contrary, it is 'the climax 

 of a complex process of growth and duplication of a multitude of cell 

 components, while inhibition of the process in vivo is clearly not due to 

 lack of nutritive substances or to any deficiency on the part of the cell.' 

 It is probable that a combination of external and internal conditions 

 is necessary. Needham^^ suggested that the 'growth-promoting factor 

 is probably no more than a right conjunction of nutrient materials and 

 the appropriate capacities for making use of them.' Synthesis of new 

 protoplasm precedes or accompanies each mitosis, so the raw materials 

 for this synthesis must be provided. It appears, therefore, to be axio- 

 matic that, for cell division to be possible, adequate nutritional condi- 

 tions for supplying energy and materials must first be fulfilled. Whether 

 the 'appropriate capacities' include a special type of stimulus over and 

 above availability of nutrients in optimal amounts, remains uncertain. 

 Baker and Carrel^^ made the distinction between substances required 

 for cell nutrition and those for cell multiplication, having already 

 shown that chick embryo extracts are an excellent source of both for 

 fibroblast tissue cultures. The problem is already clearly defined: 

 'Whether there is in embryonic extract a specific substance with a 

 function of initiating or producing cell division, or whether embryonic 

 extract simply contains the essential nutrient substances required by the 

 cells is not known. Neither is there any knowledge concerning the 

 chemical nature of the substances present in the extract which are 



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