Possible Causes of Implantation Failure in Superovulated Immature Rats12

In previous studies with immature rats superovulated with 40 IU pregnant mare’s serum gonadotropin (PMSG), neutralized 56 h later by a specific PMSG antiserum (SOV a/s rats), implantation failed to occur in 50% of animals despite the fact that free blastocysts were recovered from the uteri of 100% of rats on Day 5. To determine if the failure to implant was due to abnormalities in the blastocyst or to a failure of the uterus to provide a suitable environment, embryo transfer and decidualization studies were undertaken. Blastocysts from 4 IU or SOV a/s donors were transferred to the uterine horn on the ovariectomized side of a recipient rat and recipients were sacrificed on Day 20. Comparing recipients receiving blastocysts from either type of donor, there was no significant difference in the percentage of recipients pregnant in the transfer horn. Similarly, there was no significant difference in the percentage of transferred blastocysts recovered or in the percentage of viable fetuses. On Day 20 the mean weight of fetuses in the transfer horn was not significantly different from that in the control horn for fetuses originating from either donor. In the decidualization studies, mated 4 lU or SOV a/s rats were anesthetized on Day 5 and the antimestometrjal surface of one uterine horn was scratched with a burred needle. When animals were sacrificed on Day 10, it was found that a significantly lower percentage of SOV a/s rats than control rats were pregnant and had deciduomata formation in the traumatized horn. It is concluded from these studies that the uterus of a large proportion of SOV a/s-treated rats is unable to undergo decidualization at the normal time which may result in pregnancy loss in these animals. It is concluded that the loss of blastocysts between Days 5 and 8 in 50% of SOV a/s rats is not the result of blastocyst abnormalities and that the superovulatory treatment causes a change in the uterine environment which is not completely prevented by PMSG a/s.