I EFFECTS OF ENVIRONMENT 555 



"natural" environment, namely (so it is assumed) blood plasma, represents the 

 end to be achieved. It was thus that the "physiological" saline solutions were 

 built up, in imitation of the ionic composition of plasma. The amino acid mixture 

 recently adopted by Evans, Bryant, Fioramonti, McQiiilkin, Sanford and Earle 

 (1956) was very carefully modelled upon the pattern of amino acids found by 

 analysis in an ultrafiltrate from embryonic extract and horse serum, a biological 

 mixture which had been found to be an active growth supplement. This may be 

 a useful step, but it may be questioned whether such thinking does not somewhat 

 underestimate the synthetic and selective capacities of the cells themselves. It may 

 even turn out that the number and amounts of amino acids found in biological 

 fluids are not at all equivalent to the optimum mixture for growth. The activity 

 associated with such a mixture may well represent a balance between necessary 

 and unnecessary or even inhibitory factors. 



Nevertheless, much useful qualitative information about factors which contri- 

 bute to cell nutrition and growth has accrued from attempts to imitate natural 

 media. But sometimes too close adherence to this idea has proved misleading. For 

 example, almost all the physiological salt solutions in common use, one or an- 

 other of which forms the inorganic basis for the composition of most artificial or 

 synthetic nutrients which have been devised, are, as we have seen, imitations of 

 the inorganic composition of blood plasma. It is of course well known that cells 

 have a highly developed capacity for active uptake of certain ions; that the ionic 

 composition within a cell is very different from that of its normal environment ; 

 and that moreover most ions, both anions and cations, are not free within the cell, 

 but exist there in the form of complexes. It should not, therefore, be surprising 

 to find that, for example, although cells have a capacity to concentrate phosphate 

 from a medium low in phosphate ions, growth in an otherwise adequate medium 

 can be stimulated markedly by increasing the external concentration of inorganic 

 phosphate to a value considerably above that in normal serum (Waymouth, 1954a). 

 It is probable that one of the important contributions of the classical embryonic 

 extract to the growth of cells lies in the fact that such extracts contain a high 

 concentration of bound but easily available phosphate. This is a particvilar case; 

 the reasoning is undoubtedly of general application. 



With the above reservations in mind, we may briefly summarize the require- 

 ments, in chemical terms, of cells in tissue culture for growth. Detailed information 

 is available in the reviews of Stewart and Kirk (1954) and of Waymouth (1954b). 

 It appears to be generally accepted that growth will take place in media approxi- 

 mately isotonic with blood plasma (avian or mammalian), but a fairly wide range 

 of hypo- and hypertonicity can be tolerated by some cells. The composition of 

 individual ions can also be varied, and for some cells at least, and under certain 

 conditions, considerably higher concentrations of certain ions than are found 

 free in plasma are tolerated, or prove actually stimulatory [e.g. Owens, Gey 

 and Gey, 1956). Glucose is the usual carbohydrate supplied, and forms the 

 principle source of energy for the cells. The most important low-molecular weight 

 components other than salts and sugar appear to be amino acids, and several of 

 the B group of vitamins which are important as precursors of the metabolically 

 active coenzymes. In some instances, a purine derivative may be stimulatory; 



Literature p. 581 



