LATE EFFECTS OF IRRADIATION ON FERTILITY IN FEMALE MICE 231 



just before the onset of sterility, all suggested that the results might be due 

 simply to reduction in the number of oocytes in immature follicle stages. This 

 has turned out to be the case in both rat (Ingram, 1958) and mouse. 

 Investigations of the adult female mouse (Oakberg and Clark, 1961b) 

 have shown that the variations in effect on reproductive life-span with 

 different doses, dose fractionations, and dose-rates, are closely paralleled by 

 the amounts of cell killing in the early oocytes. This paralleHsm is also seen in 

 the highly sensitive infant mouse. Table II. The marked effect on fertility 

 of irradiation at this age (Russell et al., 1959b) is closely correlated with an 

 unusually high frequency of oocyte killing (Oakberg, 1962). 



Table II. Effect of dose-rate on survival of primary oocytes of the mouse 

 (condensed from Oakberg and Clark, 1961b)f 



•f All mice killed 72 hours after exposure. Ratios were computed on the basis of normal cells 

 counted in serial sections. 



The exact mechanism of the oocyte killing is not understood. What is 

 known can be summarized briefly. It has been amply demonstrated that the 

 cells die shortly after irradiation (Oakberg, 1960). Since the oocytes are in a 

 modified resting stage, it is also clear that cell division and DNA synthesis, 

 which are frequently invoked as playing a role in cellular death from irradia- 

 tion, can be ruled out of the oocyte response. The marked reduction in cell 

 killing that is observed when the dose-rate is lowered, indicates that the 

 initial radiation damage can be partially repaired before it leads to cell death. 



Later work may reveal some minor complexities in the effect of radiation 

 on reproductive life-span. For example, degeneration of oocytes is always 

 going on in normal, unirradiated females, and it is possible that irradiation- 

 induced depletion in the popidation of oocytes may have some effect on the 

 rate of this process. Thus, it might be expected that the rate of spontaneous 

 degeneration would decrease and so compensate, to some degree, for the 

 radiation-induced loss. This possibility is being tested in a current experiment. 

 However, even if modifying factors working in the other direction are revealed, 



