UNILATERAL OVARIECTOMY DURING PREGNANCY IN THE RAT

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Abstract
The specific time after which the pregnant rat can no longer respond to unilateral ovariectomy by increasing the number of ova shed from the remaining ovary at the post-partum ovulation has not previously been evaluated. Experiments were designed to determine this time. Such findings would elucidate, physiologically, when those follicles which are ovulated at the post-partum ovulation are determined. Rats unilaterally ovariectomized before 20.00 h on day 22 of pregnancy ovulated twice the number of eggs from the remaining ovary as would have been shed normally. This increase in the number of ova shed did not occur when 1 ovary was removed at 20.00 h on day 22 or immediately after parturition. Only those rats which had doubled the number of ova shed had an increase in the weight of the remaining ovary when compared with control animals. Unilateral ovariectomy during the last 6 days of pregnancy had no effect on the length of gestation or number of young delivered. Follicles ovulated at the post-partum estrus are probably not determined until 20.00 h on day 22 of pregnancy. Follicles are probably determined 33-36 h before being ovulated. The timing sequence of when the operation must be performed before ovulation to increase the number of eggs shed is the same in both the cyclic and the pregnant rat. Although pregnancy is a complex phenomenon, the hormonal interplay at parturition appears to be more important for follicles to mature and be shed at the post-partum ovulation, than to the mechanism of parturition itself.

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