11 PHYSICAL FACTORS 559 



the onset of cell degeneration is accelerated. Cell migration and metabolism may 

 proceed practically unimpeded after low dosages, and cells which are prevented 

 from dividing may continue to grow in size (Price-Jones and Mottram, 19 14; 

 Halberstaedter and Doljanski, 1937; Paterson, 1942; Brues and Reitz, 1951 ; Reid 

 and Gifford, 1952). For example, cells of the HeLa strain of human cervical carci- 

 noma, subjected to 1000 r, can yield a population consisting solely of giant cells, and 

 even after 10,000 r, 5-20% of the irradiated cells develop into giant cells with a 

 volume about 15 times that of the normal HeLa cell (Puck and Marcus, 1956). 

 Very high doses, of the order of 100,000 r, may be required to cause immediate 

 cessation of all cell activity, though 2000-4000 r are sufficient to cause irreversible 

 injury and ultimate complete elimination of a cell population. A review of the 

 effects of radiations upon cells in tissue cultures has been made by Stroud and 

 Brues (1954). 



{e) Effects of pH 



As with the composition of nutrient solutions, so with pH, it has been usual to 

 imitate host blood. Therefore media for mammalian and avian tissues have usually 

 been prepared at about pH 7.4-7.6. However, as the range of pH which most 

 cells will tolerate is quite wide, at least for short periods of time [cf. Lewis and 

 Felton, 1922) and as cells actively metabolizing glucose rapidly lower the pH of 

 their environment unless the medium is very well buffered, it is common practice 

 to start with a medium of rather higher pH {e.g. pH 7.8-8.0). Phenol red, which 

 in concentrations of up to about 2 mg/ioo ml is not toxic to living cells, is custom- 

 arily incorporated into tissue culture media, so that changes in pH can be easily 

 detected visually. 



Not much evidence is yet available about the true pH optima for the mainte- 

 nance and growth of different cell types, though these will undoubtedly be found 

 to vary. Adult skin epithelium has been shown (Parshley and Simms, 1950) to 

 grow best in a medium containing one part of serum or plasma diluted with two 

 parts of a solution of pH 6.8; whereas fibroblasts from adult aorta grew best in the 

 range pH 7.3-7.9 and were somewhat inhibited at pH 6.9. Growth of a mam- 

 mary tumour was achieved at the unusually high pH of 9.3 (Pikovski, 1954); 

 normal mammary gland or connective tissue cells were severely damaged or 

 destroyed at this pH. Femurs from 7 day chick embryos develop 19% more bone 

 in 14 days at pH 7.0-7.2 than at pH 7.8-8.0 (Paff, 1948). 



(/) Effects of tonicity 



Here again it has been the usual practice to simulate the "natural" environment 

 of the cells, namely plasma. And here also there is evidence that cells can tolerate 

 environments of ionic strengths, and therefore of tonicities, which vary widely 

 from the norm. Reports of the use of slightly hypotonic solutions appear in the 

 literature from the time of Carrel and Burrows (191 1). It has been common prac- 

 tice to incorporate hypotonic solutions^^.^. Pannett and Compton's (1924) 

 solution which has a total ionic strength approximately one half of that of Ringer's 

 or Tyrode's solutions — into tissue culture media. It should not, however, be 

 inferred from this that hypotonicity is the optimum condition for cells in vitro. The 



Literature p. 5S1 



