Effects of age and parity on the breeding potential of mice with one or two ovaries

Although removal of one ovary from young CBA/Ca mice and CFLP mice did not reduce the overall numbers of ova shed at ovulation, the total number of live offspring delivered during the lifespan was reduced to 65% and 50%, respectively, of control values. This reduction was due to fewer and smaller litters. The capacity of intact and unilaterally ovariectomized animals to support gestation of embryos transferred from young donors was tested to determine whether premature exhaustion of uterine function had occurred as a result of embryo overloading. An effect of breeding history on embryo survival to Day 19 of pregnancy was found after unilateral transfer, the least favourable sites for survival being the primigravid horns of ageing intact and unilaterally ovariectomized mice (0% and 1% survival, respectively, in CBA/Ca mice). The proportion of embryos surviving in multiparous horns of the one-ovary animals (24%) was greater than in horns of primigravidae and less than in horns of intact multigravidae of similar age and parity (48%). A larger proportion of ova survived in young uteri than in any of these aged horns. The results suggested that the normal decline with age in breeding potential is due to decreased uterine capacity and that the rate of loss is accelerated by both repeated embryo overloading and prolonged nulliparity, probably as a result of local factors. The decidual response was reduced in older animals, although there was no clear-cut variation with parity. Ageing uteri accumulated mass cells and macrophages, but the latter were abundant only in multiparous horns and were probably related to puerperal involutionary activities. There was no evidence that these changes in cell number or response were responsible for decreased gestational potential in ageing animals.