MAMMALIAN CELL GROWTH IX TISSUE CULTURE 189 



importance. The applications of these considerations to the problem of 

 treatment of cancer by ionizing radiation also appears to be assum- 

 ing practical importance ( Scott, 1958; Puck, 1960c; Elkind and Sutton, 

 1959). The combination of the intrinsic radiosensitivity of the cellular 

 reproductive apparatus, the contribution of back-scattering to the ac- 

 tual dose received by diflFerent cell components (Hood, 1960), the 

 oxygen tension in equilibrium with the cell, the number of malignant 

 cells that must be sterilized before a tumor can be considered eradi- 

 cated, and the eflFects due to the presence of radiation cell-protective 

 compounds (Morkovin and Puck, 1958)— all these now appear to be 

 readily measureable parameters whose effects will help to elucidate 

 greatly the course and degree of hopefulness of cancer therapy. The 

 extent to which the host's reaction, as by immunological defense mech- 

 anisms, may also contribute to the success of radiation procedures re- 

 mains to be delineated. It is important to note that, whereas, to a first 

 approximation, most mammalian cells so far studied quantitatively have 

 similar, high sensitivities to reproductive death by ionizing radiation, 

 at least some differences do exist. Even a small difference in D° value 

 may have a profound effect on the outcbme of high doses of radiation. 

 Hence it now becomes of great importance to measure as precisely as 

 possible the effects of the various parameters that can affect cell 

 survival. 



From a consideration of several theoretical aspects of the survival 

 curve of the S3 HeLa cell it was deduced that the principal seat of the 

 lethal action lay in the chromosomes. Moreover, because the survival 

 curve was multiple-hit in form, it was postulated that death must re- 

 quire, at least in a significant proportion of such events, interaction be- 

 tween simultaneously hit chromosomes. Hence it was suggested that 

 chromosomal bridges, known to occur in other living forms, may be an 

 important concomitant of irradiation in mammalian cells at these dose 

 levels, and may make an important contribution to the cell-lethal 

 mechanisms. Direct observation of such irradiated cells by means of 

 time-lapse cinephoto micrography in our own laboratory and by other 

 investigators (Whitfield et at., 1959) has amply borne out this pre- 

 diction. Chromosome bridges are observed in an appreciable proportion 

 of the cells irradiated with doses two to five times the D° value. 



Because a number of investigators have questioned the chromo- 

 somal role of the X-ray lethal process, it may be worth \\ hile to sum- 

 marize the evidence. ( 1 ) The cell-lethal dose parallels closely the chro- 

 mosome-damaging dose. The parallelism is not complete, because of 

 the complexity of the process involved (Puck, 1960), but the cor- 

 respondence is more than sufficient for chromosomal damage to con- 

 stitute the primary step leading, by a variety of pathwa\s which have 

 been described, to cell-reproductive death. (2) Demonstration of the 



