Limitation of Fetus Number in the Rat, Mouse and Rabbit

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Abstract
Seventy-four female rats, 72 female mice and 30 female rabbits were used as experimental animals. Rats were sham operated, had one oviduct sectioned or were unilaterally ovariectomized (UO). Mice and rabbits were either intact or UO. One-half of the rabbits in each of these groups were superovulated. In rats and mice corpora lutea (CL) and fetus number were counted on days 7 and 17 post coitus and the rabbits on days 8, 18 and 28. UO resulted in increased numbers of CL in the remaining ovary to about double the number on a single ovary of the control animals. This resulted in a near equal number of potential fetuses in the control and UO groups. Superovulation in the rabbit increased CL in both intact and UO animals, but the total per animal was about twice as great in the intact animals as in the UO group. Based on the number of fetuses per animal, UO animals of all three species carried as many fetuses in one functional uterine horn as did the intact animals in two functional horns. This resulted in a near doubling of offspring per uterine horn. The one exception to this case is in the superovulated rabbits at day 8. It appears in the rabbit at day 8 that there is a limitation of the number of fetuses by available uterine space. Thereafter in the superovulated rabbit and in animals of all three species it appeared that fetus number was controlled not by the amount of uterine space available, but by a more systemic limitation inherent in the female. This is evident since unilateral and bilateral pregnancies produced equal numbers of fetuses. Copyright © 1970. American Society of Animal Science . Copyright 1970 by American Society of Animal Science

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