THE FATE OF EMBRYOS TRANSFERRED INTO THE UTERUS

Purpose: The purpose of this study is to elucidate the fate of embryos transferred into the uterus. Methods: Implantation rates were compare d between synchronous and asynchronous intrauterine embryo transfers i n mice. Then transferred embryos were recovered from the uterus and ex amined morphologically 24 hr after transfer. Moreover, the same transf er-recovery experiments were performed after ligation of the uterus tw o-thirds from the uterotubal junction immediately before transfer. Res ults: The implantation rate was high (58.4% per embryo, 87.5% per reci pient) when blastocysts were transferred into the uteri of recipients of pseudopregnant Day 4, but it was very low (0-2.4% per embryo, 0-8.3 % per recipient) when blastocysts were transferred to recipients of ps eudopregnant Day 2, or two-cell embryos into recipients of pseudopregn ant Days 2 and 4. When transferred embryos were recovered from the ute rus 24 hr after transfer, few embryos (1.4%) were recovered in asynchr onous transfers, while 66.3% of the embryos were recovered in synchron ous transfers. However, from recipients in which the lower portions of uteri were ligated, embryos at more advanced stages were recovered ev en in asynchronous transfers. Conclusions: These results suggest that discharge from the uterus is one of the major causes of the low implan tation rate for asynchronous transfer.