Abstract
The hostβtransplant 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.