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SUMMARY The oocytes in the ovaries of virgin and multiparous mice (A, CBA, RIII and CBA × A strains) have been classified and counted. The ages of the virgin mice ranged from 0 (the day of birth) to 933 days and of the breeding mice from 93 to 735 days. In each strain there is a continuous gradual decline in numbers of oocytes which can be expressed by regression equations of the form: log y = a + b (x − [unk]), where y = number of oocytes and x = age in days. The strains differ in the stage of development reached at birth, in the total numbers of oocytes they contain and in the rates at which the oocytes are lost (given by the regression coefficients b). CBA mice lose their oocytes most rapidly and this is the only strain in which the ovary became totally depleted of oocytes long before death. The proportion of oocytes classified as normal, on histological grounds, falls during the first few weeks of life to a figure of 50–60%, around which it remains throughout the rest of life. The strain with the highest proportion of normal oocytes was the one in which the decline in total numbers occurred most slowly. Breeding has no significant effect on the rate at which the total number of oocytes declines. Figures for the levels of fertility (in terms of litter size at birth) throughout the reproductive lifespan follow the accepted pattern of rising productivity for the first few litters followed by a gradual decline until the mice become sterile. The levels of fertility differ between the strains but cannot be related either to the total number of oocytes, the rate at which they decrease, or to a decline in numbers of Graafian follicles or corpora lutea. It is suggested that the explanation for the decline in fertility towards the end of the reproductive lifespan is more likely to be found in defects in the hormonal control of the ovary or in the uterine environment rather than in the loss of oocytes per se.

The technique of ova transplantation was used to study the causes of the decline in litter size in ageing mice. Only 14% of morulae and blastocysts obtained from the uteri of young donors survived to term when transplanted into the uteri of old hosts (13 to 24 months of age), whereas 48% survived in young hosts (2 to 7 months of age). A slightly higher percentage of grossly abnormal ova was recovered on the 4th day of pregnancy from the uteri of old mice than from young mice (13% compared with 5%) However, normal-appearing morulae and blastulae from old mice survived as well as those from young mice when transferred into uteri of young hosts. It was concluded that the initial decline in litter size in aged mice is probably due to an unfavourable uterine environment.

1. A surgical technique for the orthotopic transplantation of the ovaries in mice is described and its immediate and long-term difficulties are discussed.2. The shortened reproductive lifespan of mice which have received such grafts is due primarily to the loss of oocytes during the immediate postoperative period before a new blood supply has developed. Figures are given for the loss, which may extend to between a half and three\x=req-\ quarters of the total complement of oocytes.

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