Survival of bisected rabbit morulae transferred to synchronous and asynchronous recipients

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Abstract
The rabbit was used as a model to test the concept that temporal asynchrony is required to establish physiological synchrony when embryos are bisected to produce demiembryos. In preliminary studies with intact embryos it was confirmed that embryos harvested on days 2,3,4, or 5 (day 0 = day of breeding) can be transferred with ± 1 day of asynchrony to the uteri of recipient rabbits. Three experiments were conducted with bisected embryos. In experiment 1, 192 bisected and 194 control day 3 embryos were transferred to uteri of day 2,2.5, and 3 recipients (ovulated 0, 12, and 24 h after the donors), with 14% of the bisected and 39% of the intact embryos (P < .05) resulting in young. Only 4% (2/48) of the day 3 bisected embryos vs. 39% (P < .05) of the intact day 3 embryos survived in the uteri of day 2 recipients. In experiment 2, day 3 bisected and intact embryos were transferred to the oviducts of day 3, 3.5, or 4 recipients, the speculation being that the oviduct might provide a more neutral enviroment than the uterus. However, embryo survival was very low, except for the intact embryos transferred to synchronized recipients (42% young born). In experiment 3, 150 intact and 162 (81 pairs) bisected day 3 embryos collected from uteri were transferred to uteri of day 2.5, 3.0, and 3.5 recipient does. Significantly more pregnancies (100% vs. 47%, P < .01) and young born (56% vs. 19%, P < .01) resulted from intact embryos than from bisected embryos, irrespective of the uterine age. Asynchrony (±0.5 days) did not affect pregnancy rate or young born from day 3 intact embryos, but bisected embryos are more sensitive to uterine or oviductal environment than whole embryos; survival was higher (P < .05) when they were transferred to day 2.5 recipients.