DAMAGE TO REPRODUCTIVE CAPACITY OF HUMAN CELLS 185 



about four hours of incubation at 37°C more tliaii li!» per cent of the 

 cells adhered sufficiently to the bottom of the culture dishes for the 

 experiments to be carried out. During the irradiation experiments the 

 cells were uiaintained at room tem])erature, 18 to 22°C. The cultures 

 were then placed in the incubator and the cells allowed to grow and 

 multiply for U days at 37°C. Medium was replaced once every five days. 

 After 14 days the cells were stained in situ and the number of clones of 

 more than fifty cells was counted. The number of clones has been taken 

 to represent the number of surviving cells. In each experiment the mean 

 value was taken of at least three culture dishes each of which had re- 

 ceived the same dose. Plating efficiencies of unirradiated cells ranged 

 from 50 to 100 per cent, with an average of about 80 per cent. The 

 fraction of cells surviving irradiation was calculated as a percentage 

 of the unirradiated controls in the same experiment. 



Irradiations were carried out with a-particles from 2i0Po (LET v 

 170 keV///), 20 kV X-radiation, unfiltered except for a layer of culture 

 medium 1mm thick, (HVL 0-05 mm Al, LET ^ 6keV///), 200 kV 

 X-radiation filtered by 1-5 mm Cu (HVL 1-9 mm Cu, LET x ^-5 

 keV//,0 and /3-radiation from ^oy (LET x 0-3 keV//0. Because of the 

 low penetrating power of a-particles from 'lopo, which have an 

 energy of 5-3 MeV and a range in water of 37^, the cells were cultured 

 in special dishes with a "Melinex"t bottom about 6/t thick. This thin 

 melinex permitted irradiation of the cells from outside the culture 

 dishes a few hours after plating, when the majority of the cells adhered 

 to the bottom. These Melinex dishes were used in all experiments and 

 were found to support reliably the growth of each surviving cell into 

 a macroscopic clone, even after irradiation which has been reported 

 to give rise to toxicity in plastic surfaces (Morkovin and Feldman, 



1959). 



Dose measurements were carried out with various dosimeters. The 

 dose of a-radiation delivered to the cells was determined by counting 

 the number of particles passing per minute through 1 sq. nnn, by 

 measurement of the total current in a large ioinzation chamber and 

 with an extrapolation chamber. The dose of |8-radiation was measured 

 w ith the extrapolation chamber. The dose of 200 kV X-radiation was 

 measured with a Baldwin "Substandard" ionization chamber and with 

 the extrapolation chamber. The dose of 20 kV X-radiation was measured 

 with a Philips ionization chamber type 37483/01, suited for HVL 

 0-02- 1-5 mm Al. 



Details of the irradiation techniques and dosimetry are given else- 

 where (Barendsen and Beusker, 1960). 



t Polyethylene terephthalate. Imperial Chemical Industries T.imited, Herts., England. 



