THE RELATIONSHIPS BETWEEN AGE, NUMBERS OF OOCYTES AND FERTILITY IN VIRGIN AND MULTIPAROUS MICE

Abstract

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 .