554 GROWTH IN TISSUE CULTURE 6 



{b) Chemical factors 



Tissue culture started as a highly empirical practice, and has only gradually 

 moved towards being a technique vmder much fuller experimental control. No- 

 where is this more clearly exemplified than in the development of the media used. 

 The complex biological fluids used at first — lymph, serum, plasma, tissue extracts 

 —still form the nutritional basis for most cultures. But great advances have been 

 made, through the early steps from homologous to heterologous sera and extracts, 

 through the stages of simplification of the biological media by the use of fraction- 

 ation procedures and through the building up of defined nutrient solutions, to a 

 much fuller understanding of many specific nutritional requirements of living 

 cells for growth. 



Nutrition is a term nearly as elusive as growth when exact definitions are sought. 

 The best that can be done is to define it as the act of supplying whatever is neces- 

 sary for the growth, formation or proper condition of (in the present context) the 

 cultures. Perhaps because of the indefiniteness attached to both "growth" and 

 "nutrition", there has been some confusion between the two concepts. Growth 

 does depend upon the adequacy of the environment to support it; growth is 

 closely associated with, and in some cases is synonymous with, synthesis of new 

 cellular substance; and the principal environmental factors upon which this syn- 

 thesis depends are, by definition, nutritional. In other words, proper nutrition is a 

 necessary, but not a sufficient condition for growth. With this confusion has been 

 associated the somewhat unspecific and often very unhelpful concept of the "growth 

 promoting substance". Assuming a set of conditions sufficient for maximum growth 

 (and for any cell type there may be several diflferent systems providing this possi- 

 bility), alteration or omission of one single factor may make the environment less, 

 or not at all, able to support growth. This missing or altered condition (which 

 may be, for example, a single nutrient substance, or a change in some physical 

 factor such as temperature) may then be called a "growth promoting factor". 

 The search for specific growth promoting factors in biological nutrients has been 

 fraught with many difficulties, and these have largely arisen because the multi- 

 variant nature of the tissue culture system has not been clearly enough appre- 

 ciated. So the search for the variously named "trephones", "embryonin" etc. has 

 been little more fruitful than the search for the philosophers' stone. Because it has 

 not been understood that any one of many factors could be a limiting one, appar- 

 ently contradictory statements have appeared; e.g. here, that heated embryonic 

 extract is non-growth promoting; there, that it is active in this respect. Or, that 

 growth depends on a high molecular fraction from embryonic extract and a low 

 molecular one from serum; and that the high molecular fraction from embryonic 

 extract is inactive, whereas the low molecular fraction from embryonic extract and 

 the high molecular fraction from serum is active. These statements are all prob- 

 ably true in relation to the rest of the systems used in each case. It is not appro- 

 priate to speak of "the" requirement of any cell culture for any single nutrient, 

 without defining the rest of the system, nor is it proper to search in a biological 

 nutrient for "the" growth promoting factor. 



Much work aimed towards producing an environment optimal in chemical 

 terms for the growth of cells has been based on the concept that simulating the 



