II PHYSICAL FACTORS 557 



Parkes, 1954; Pomerat and Moorhead, 1956). The most effective media are those 

 in which ice crystal formation is minimized. Cooling and thawing in saline solu- 

 tions, or in sucrose, permits little or no survival. Serum is a better protective 

 medium, but the most satisfactory suspension fluids contain glycerol or ethylene 

 glvcol. Luyet and Gonzales (1951) obtained abundant outgrowth from chick 

 heart which had been frozen rapidly to — 195° in 30% ethylene glycol, and rapidly 

 thawed. Ovarian granidosa cells survived slow cooling better than rapid freezing: 

 50-70% of these cells svirvived slow freezing to — 79° in serum ; the survival was 

 even better in serum plus glycerol (Smith, 1952). 



Different types of cell respond differently to brief exposure to low temperatures 

 (thermal shock). Pomerat and Lewis (1953) exposed chick skin and skeletal muscle 

 to temperatures ranging from — 3.4° to — 39-2° by rapid cooling and immediate 

 thawing, and prepared cultures from the treated fragments of tissue. The cells 

 frozen either in Gey's balanced salt solution or in serum showed different lower 

 temperature limits for survival. Muscle fibres emigrated after exposure to — 18° 

 or — 20°; skin epithelium tolerated — 30°, and spindle cells resisted temperatures 

 of near — 40°. Similarly, there was a difference, though perhaps not very signif- 

 icant, in resistance to low temperatures of human newborn epithelium ( — 24.4°) 

 and adult human skin ( — 20.5°). Human spindle cells, like those of the chick, 

 could withstand lower temperatures ( — 30°). 



Mitosis can apparently persist at much reduced temperatures. Spear (1928) 

 observed that the duration of mitosis in chick cells was doubled by a 10° drop in 

 temperature (38°-28°), and was still further delayed at lower temperatures. 

 Nuclear divisions were still seen by Bucciante (1927-28) at 0°, though without 

 cytoplasmic division, so that multinucleate cells were formed. The fact that cul- 

 tures kept at temperatures a few degrees below physiological {e.g. 30°-34°) have 

 a lower metabolic activity and mitotic rate has been put to practical use by Gey, 

 Hanks and Barrett (1948) for maintaining cultures with a minimum of care. 

 In an appropriate medium, chick cells were kept for 60 days at 30° without renew- 

 al of the medium (Hanks, 1948). 



Tissues from cold-blooded animals can grow at rather a wide range of temper- 

 atures, and an interesting study has been made of the effect of variations in tem- 

 perature on the growth of cultures of frog kidney carcinoma by Lucke, Berwick 

 and Nowell (1953). Using Ebeling's (1921) area measurement method, the 

 total new growth area was plotted against time, in four temperature ranges 

 (20°-35°). The temperature coefficient of growth, Q_iq, was computed from the 

 formula : 



, ^ (log K, — log K.) 



log 0,10 = \t X ^° 



where : K^ = rate of growth at the higher temperature 

 K2 = rate of growth at the lower temperature 

 A ^ = difference of temperature between two measurements. 



Q.]o was found to decrease slightly with time over a 2-7 day growth period. The 



average value of Q^jq was 2.50 (range 1.9-3.9). 



Literature p. 581 



