Inequality in function of the right and left ovaries and uterine horns of the mouse

Inequality in function of the left and right ovaries and uterine horns of mice was evaluated in three separate experiments. In Exp. 1, the effect of position in the reproductive tract on various reproductive characteristics was evaluated in 158 pregnant hybrid mice. Ovulation rate, number of fetuses, total fetal weight and total placental weight were higher (P less than 0.05) on the right than the left on Day 18 of pregnancy (vaginal plug = Day 1). In Exp. 2, the effect of previous sham or unilateral ovariectomy (right or left) in mated Swiss-Webster mice was compared with unoperated mated controls (N = 17-24/treatment). In control mice, ovulation rate, total fetal weight and ovarian weight were higher (P less than 0.05) on the right than left side. Surgery (sham or unilateral, ovariectomy) decreased (P less than 0.005) ovulation rates, number of fetuses, ovarian weights, total fetal weight and total placental weight on Day 18 of pregnancy. Unilateral ovariectomy decreased (P less than 0.05) ovulation rates and ovarian weights more than did sham operation. Ovulation rates were higher (P less than 0.01) when the left ovary was manipulated or removed rather than the right ovary. For Exp. 3, pairs of 8 hybrid mouse embryos each (morulae and blastocysts) were surgically transferred to the left and right uterine horns of the same (bilateral, N = 15) or different (unilateral, N = 28) Swiss-Webster recipients. In almost all incidences, embryo survival (to Day 18 of pregnancy) was twice as high (P less than 0.05) in right than left uterine horns. We conclude that the left and right ovaries and uterine horns are not equal in function in Swiss-Webster and a hybrid strain of mice.