In cryopreserved rat embryos, survival rates obtained in vitro are not always consistent with the rates obtained in vivo. To determine the optimal conditions for in vivo development to term, rat embryos at the 4-cell, 8-cell, and morula stages were vitrified in EFS40 by a one-step method and transferred into oviducts or uterine horns of recipients at various times during pseudopregnancy. Vitrified and fresh 4-cell embryos only developed after transfer into oviducts of asynchronous recipients on Days −1 to −2 of synchrony (i.e., at a point in pseudopregnancy 1–2 days earlier than the embryos). Approximately half the vitrified embryos transferred into oviducts on Day −1 developed to term, but only a minority of embryos, whether vitrified (10%–34%) or fresh (24%–33%), transferred at later times did so, suggesting that this may not be the most suitable stage for cryopreservation. Very few 8-cell embryos, either vitrified or fresh, developed when transferred into oviducts on Day 0 to −0.5. However, when transferred into uterine horns, high proportions of vitrified 8-cell embryos (∼63%) developed to term in reasonably synchronous recipients (Day 0 to −0.5) but not in more asynchronous ones (6%; Day −1). A majority of vitrified morulae also developed to term (52%–68%) in a wider range of recipients (Days 0 to −1), the greatest success occurring in recipients on Day −0.5. Similar proportions of vitrified and fresh 4-cell embryos, 8-cell embryos, and morulae developed to term when appropriate synchronization existed between embryo and recipient. Thus, vitrification of preimplantation-stage rat embryos does not appear to impair their developmental potential in vivo.
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