LEUKAEMOGENIC EFFECT 33 



DISCUSSION 



gray: Have you any idea what the probability of survival of any of your cells was which 

 took up the thymidine? Because if there is a specific initiating event it is obviously 

 being expressed only when the cells divide. 



cottier: We have but very little information on this as far as long-term effects are 

 concerned. Tonna, Shellabarger and Cronkite (unpublished data) found that 25yLic of 

 tritiated thymidine given to newborn rats did not influence their growth-rate up to the 

 age of 9 months. At this time there were stUl some labelled cells found in various tissues. 

 Our mice treated with tritiated nucleosides as mentioned above seemed to behave quite 

 normally. They did not undergo a weight loss comparable to that observed in the 

 «"Co-y-irradiated group. One mouse died accidentally 3 weeks after the last injection of 

 tritiated thymidine; the thymus still contained a number of labelled cells. The other 

 material is not yet processed. 



UPTON: May I ask Dr. Cottier what proportion of cells in the bone-marrow and thymus 

 were labelled? A number of workers have sho\vn now that, to induce thymic lymphoma, 

 whole-body irradiation was necessary. If one gives a pulse of thymidine one doesn't 

 succeed in labeUing all dividing cells although if you give several pulses you can get that 

 later. 



COTTIER: In order to establish a meaningful labelling index one should be able to recognize 

 specific dividing cell classes as such. With respect to the lymphatic tissue this in itself is 

 a difficult problem. After a single injection of tritiated thymidine the "large lymphoid 

 precursors" in the thymus or in germinal centres of lymph nodes and spleen show a 

 labelling index rangmg from 50 to 70%. Thus it seems very probable that a number of 

 potential stem-cells were left unlabelled by this method. On the other hand there is 

 evidence that labelled cells continue to proliferate even after doses of tritiated nucleo- 

 sides such as given here. 



lamerton: Would it not be true, Dr. Cottier, that almost aU the cells would take up 

 labelled cytidine and only a proportion labelled thymidine? Knowing the range of the 

 beta particle from cytidine quite a proportion of the nuclei of these cells would not have 

 been irradiated. 



COTTIER: The situation after administration of tritiated cytidine is indeed difficult to 

 evaluate. Disregarding the fact that a fraction of this labelled precursor contributes to 

 DNA synthesis, there are still other possibilities of nuclear irradiation since 



(1) at least a good portion of macromolecular RNA is formed in the nucleus and sub- 

 sequently transferred to the cytoplasm, and 



(2) the outer shell of the nuclear cliromatin may be irradiated by tritium located in the 

 adjoining cytoplasm. 



BACQ: Is it logical, or not, to suppose that precursor cells, which do not take up tritiated 

 thymidine, are those which normally synthesize much larger amounts of thymidine? 

 So that when you measure the percentage of cells which take up thymidine what you 

 have actually shown is the inhomogeneity of the biochemical behaviour of these cells. 

 cottier: The so-called pool of DNA-precursors naturally available within the cells is 

 considered by most authors to be rather small. But there is little knowledge about the 

 actual size of this pool in specific ceU types. By continuous infusion of tritiated thymidine 

 one can label practically 100% of dividing cell classes. 



MULLER: Isn't it true that some cells cannot incorpotate thymidine into their DNA? 

 cottier: It has been reported that in some mouse leukaemias the neoplastic cells may 

 lack the enzyme that converts thymidine into its monophosphate (Shooter, Bianclii and 



