Discussion 207 



Spiegelman : Why was that tried ? 



Hollaender: There were several reasons: (1) Harrington and Koza 

 (1951, Biol. Bull., 101, 138) working with grasshopper neuroblasts, 

 found that the cells swelled almost immediately after treatment with 

 100 r or more of X-rays. This suggested a radiation-induced change in 

 osmotic-pressure relationships in the cells. The cells looked as they 

 would do had they been placed in medium hypotonic to them. 



(2) The radiation-induced "reversion" of middle and late prophase 

 neuroblast chromatin to an interphase condition, a primary cause of 

 mitotic inhibition in this cell, resembles the "disappearance" of chromo- 

 somes produced when cells are placed in solutions hypotonic to them. 



(3) Gaulden found that the chromatin of telophase, interphase and 

 early and middle prophase cells could be made to resemble chromatin of 

 late prophase by placing the cells in culture medium hypertonic to them. 

 This change occurred within seconds and was accompanied by an 

 accelerated mitotic rate (Gaulden, M. E. (1956), "Visible characteristics 

 of living interphase and mitotic chromatin in the grasshopper neuroblast 

 and the effects of abnormal toxicity on them." In manuscript). 



These observations together with those of Sugiura (1937, Radiology, 

 29, 352) who found growth capacity of irradiated tumour fragments to 

 be increased when placed in hypertonic solutions, led Gaulden to test 

 the efficacy of hypertonic medium in counteracting radiation- induced 

 reversion of chromosomes, which results in mitotic inhibition. 



Lajtha: Dr. Howard mentioned the possible interphase killing effect of 

 5000 r, which I think is a very important point. It undoubtedly kills 

 some cells, you can see them dying in certain cultures. However, the 

 numbers are relatively low. We repeated the experiments with 1000 r 

 and I think we have indication for an interphase effect. The G^ period is 

 very long, or relatively long in the bone marrow cells, of the order of 20 

 hours or more. If, therefore, the cells would be damaged only during 

 mitosis, then for a considerable time afterwards undamaged G^ cells 

 would enter and go through their synthetic period making the normal 

 amount of DNA. We find on the other hand that even after 1000 r all 

 the cells which enter the synthetic period produce only a fraction of the 

 normal amount of DNA. The grain counts instead of the normal 60-80 

 are of the order of 10 or less, and we don't see any appreciable number of 

 dying cells after 1000 r. This rather suggests that the whole G^ period is 

 damaged by radiation. The one difficulty is that 1000 r has a direct 

 effect on the synthetic period as well, i.e. it will stop DNA synthesis then 

 and there. I think we must repeat these experiments with 300 r or less, 

 as you did with 150 r. 



Howard: These effects ought to be separable since the dose effect is so 

 widely separated. One can have a big mitotic delay with 100 r. 



Lajtha: A dose of 150 r did not inhibit the synthetic period in the bone 

 marrow cells, just as in your experiments with bean roots it did not 

 inhibit the S period. 



Swanson : May I ask your opinion about this first effect of radiation 

 in terms of DNAse, i.e. where the chromosomes become sticky; is there 

 any clue as to what is actually happening there ? 



