Host‐transplant relationship of cultured mouse embryos

The host‐transplant relationship of cultured mouse embryos was studied in mice of strain 129/SvSl. Immature females were superovulated and embryos obtained at the four cell stage were cultured in vitro for 48 hours using Brinster's technic. At the conclusion of the culture period, 80.9% had developed to blastocysts. These were transplanted by direct uterine insertion into one to four day pseudopregnant nulliparous hosts three to seven months of age. Transplants to a one day host are lost 24 hours after transplant. The trans planted four day old embryos implant and develop into fetuses most success fully in two day pseudopregnant hosts. The blastocysts are retained in the uterine lumen for up to 72 hours before implantation. Transplantation to three day hosts allowed fetal development but at a much lower percentage than with two day hosts. Blastocysts were found to be retained in the uterine lumen for up to 48 hours. A very low percentage of transplants to four day hosts did implant, but no fetal development occurred. It is apparent that a critical period of adjustment and interdependence between embryonic development and uterine receptivity is required before implantation of the cultured embryo can proceed. Embryos which implant develop according to post implantation age, not embryonic age, and are delivered according to the normal uterine gestation time.