EXPERIMENTAL ANALYSIS 



In a discussion of the factors involved in wound healing, Carrel^'' 

 recalls that Weigert denied the power of any agent exterior to the cells 

 to stimulate their proliferation, but Virchow thought that inflamma- 

 tory growth was stimulated by irritants which incited the cells to 

 proliferate, and Welch regarded these irritants as chemical in nature. 

 Carrel argues that as there is no evidence for inhibitory agents able 

 to overcome an inherent tendency to growth, direct growth-promoting 

 action, such as that associated in vitro with embryonic and leucocytic 

 extracts, must be responsible for stimulating proliferation. 



Cell division and differentiation 



Whereas in the early stages of embryonic development all the cells of 

 an organism are able to divide, as differentiation proceeds, this ability 

 may be reduced or lost as the result of a kind of 'physiological division 

 of labour' (Fischer^^^^ Xhe seeming antagonism or incompatibility 

 between proliferation and functional differentiation has been recognized 

 at least since 1905 (Schaper and Cohen^^) and has been frequently re- 

 stated, e.g., by Strangew^ays,^^ Lorrain Smith,^^ Reimann,^* Need- 

 HAM^*^ and NiCHOLSON.^^ The relationship between cell division and 

 differentiation is discussed by Dawson,^^ especially in the light of the 

 views of Bloom^' and Weiss. ^^ Bloom states the thesis that 'cellular mul- 

 tiplication and cellular differentiation are distinct processes, each of 

 which usually takes place only in the absence of the other'. The empha- 

 sis may differ, as when Fischer and Parker^^ state that 'reproductive 

 quiescence is an important factor in differentiation', while Weiss^^ puts 

 it: 'In general it can be said that, the more specialized a cell is in its 

 structure and function, the less apt it is to divide.' There is no hard and 

 fast rule. Though always stated with cautious reservation, the conclusion 

 is invariably the same. 'Cellular differentiation and cellular multiplica- 

 tion are two processes which, if not strictly mutually exclusive, are 

 nevertheless markedly antagonistic in their tendencies' (Weiss^^) and 

 'proliferation does not interfere with differentiation, but it does 

 impede the elaboration of certain manifest products of differentiation' 

 (Weiss^). Daw^son,^^ while agreeing with Weiss that 'a differentiating 

 cell . . . gradually loses its capacity to divide and factors promoting 

 differentiation automatically reduce proliferation', points out the 

 fallacy of the converse supposition, found in the literature, that the 

 apparent inability of certain cells to divide can be accepted as primary 

 evidence of their high degree of specialization, and he emphasizes the 

 effect of the environment in determining whether or not a cell will divide. 

 He states the problem, that 'we must determine whether the incapacity 

 to divide is due to intrinsic or extrinsic factors, or to an interaction 

 between them', and suggests that the capacity to divide may simply be 

 suppressed by unfavourable environmental conditions. A similar idea is 



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