224 Discussion 



Forssberg: Cycle time may be taken as approximately constant for a 

 48-hour observation period in controls. In irradiated samples mitosis 

 disappears for about 15-20 hours. What the cycle time may be when 

 mitosis appears, I cannot say. 



Howard: Dr. Forssberg, since you have done experiments showing 

 that labelled nucleic acid of dead cells can appear in the nucleic acids of 

 growing cells in the tumour, and apparently can also stimulate division, 

 do you regard this as something which might happen in a tumour which 

 is given heavy doses of irradiation in which there must be a great deal 

 of cell death, and do you think this would be a factor in the treatment 

 of tumours with radiation ? These dead cells are doing something to the 

 metabolism of the living ones, in the way of stimulating growth; what 

 significance has this in therapeutic treatment of tumours ? 



Forssberg: I think that in therapeutic treatment doses should be 

 kept as high as possible in order to kill as many tumour cells as possible. 

 In ascites cells these effects appear with fairly low doses. It seems to me 

 that the increased labelling of DNA which Dr. Kelly found 2-3 days 

 after irradiation could be caused by transfer from irradiation-killed cells. 



Howard: This seems quite a possibility in many irradiated tissues. 



Latarjet: Dr. Howard's question impels me to say a few words about 

 the mysterious effect which Dr. Delaporte observed in 1949 after u.v. 

 irradiation and which she has recently investigated after X-irradiation 

 in my laboratory. She irradiated Esch. coli bacteria with about 80 kr., a 

 dose which apparently leaves about 10'* colony-forming cells when a 

 heavy inoculum is plated on agar. If one looks at the plate under the 

 microscope during incubation, one observes at the early origin of a 

 colony not a single growing cell but a rather large number of enlarged 

 growing cells undergoing a few divisions. Dr. Delaporte's idea is that 

 once a surviving cell has started growing, surrounding "dead" cells, at a 

 short distance of less than 100 microns, are induced into growth. 

 Restorability would last a short time, thus limiting the process which 

 otherwise would expand to the whole plate. This restoration would be 

 very effective between clumped cells. As a matter of fact, after u.v. at 

 least, once a cell starts growing within a clump, all the cells of this 

 clump start growing too. 



I must add that what is to me the fundamental question has not yet 

 been cleared up, i.e. whether there is induction to cell enlargement 

 followed by a few divisions (delayed death instead of immediate death), 

 or to true restoration i.e. to indefinite cell multiplication. This question 

 may be answered by distinguishing the restoring cell from the eventually 

 restored ones. I am now using two mutants of Esch. coli for this purpose. 

 The experiments have just begun. The only thing I can say at the 

 moment is that, under my experimental conditions, using a high dose of 

 80 kr, the phenomenon of "neighbourhood restoration", if it does exist, 

 is rare. 



