FATE OF 2 DIFFERENT STAGES OF EMBRYOS TRA NSFERRED TO AN IDENTICAL RECIPIENT IN MICE

To define the developmental fate of two different stages of embryos ex isting together in the mouse reproductive tract, different stages of f resh and frozen-thawed embryos were transferred separately into the ov iducts of identical recipients. ICR and FvB embryos were flushed from the oviducts and/or uterus of superovulated females on Day 1 (2-4-cell stage) and Day 2 (8-cell-morula stage) of pregnancy. Day 1 embryos we re transferred separately to the right or left oviduct of recipients, while the other oviduct received Day 2 embryos. There were no signific ant differences between Day 1 and Day 2 embryos with respect to the nu mber of implantation sites and live fetuses in either the fresh or fro zen-thawed embryos. These results concerning to the embryonic stage we re similar to those of control experiments, in which Day 1 or Day 2 em bryos were transferred to both right and left oviducts of recipients. Furthermore, no strain differences were observed in this study. No dev elopmental retardation or anomalies were observed in fetuses derived f rom either Day 1 or Day 2 embryos. The embryo transfer in this study r evealed that differences in developmental stage at preimplantation wer e synchronized by the maternal uterine environment.