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V. E. Papaioannou

Summary. Mouse blastocysts transferred to the oviducts of immature females entered a period of diapause from which they could be activated by culture in vitro or by transfer to pseudopregnant recipients. In the tract of immature females, embryos hatched from the zona pellucida, increased cell number to a maximum that is comparable to that of blastocysts delayed by ovariectomy, and some moved to the uterus. Viability of blastocysts retained in the non-progestational, immature tract remained high for 4 days but dropped after 5 or 6 days. This new method of producing a delay in implantation offers precision in determining survival and viability rates and in determining the requirements of diapausing embryos.

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V. E. Papaioannou and K. M. Ebert

Summary. The in-vitro culture of fertilized 1 -cell mouse embryos to the blastocyst stage is associated with subsequent decreased viability. In this study, 1-cell embryos were cultured for 3 days in the reproductive tract of immature female mice as an alternative to in-vitro culture. Embryos that spent 3 days in immature females were developmentally more advanced, had higher cell numbers and better viability, as measured by development to mid-gestation, after transfer to pseudopregnant recipient females than did embryos maintained for the same period in culture. Embryos that developed in immature females had lower cell numbers but comparable rates of development and subsequent viability when compared with embryos transferred to synchronous pseudopregnant females for the same preimplantation period. The immature mouse oviduct is therefore a suitable alternative environment to in-vitro culture or a pseudopregnant host for complete preimplantation development and has the additional advantage that synchrony between embryo and temporary host is not necessary. This method will allow for evaluation of manipulation procedures while maintaining viability before the embryos are finally committed to a foster mother for development to term.