Uterine proteins and the activation of embryos from mice during delayed implantation

Ovariectomy-induced delay of implantation was used to study the role of the uterine environment in controlling implantation in mice. Labelling studies in vivo showed that uterine protein synthesis and secretion is maximal 2-5 h and 24-30 h after the oestradiol injection which initiates implantation. Embryos removed from uteri 5,12 or 30 h after oestradiol injection were able to transport and utilize precursors of nucleic acids and proteins in short-term cultures at the same rate as normal embryos, although "delayed" embryos had low levels of activity. These results suggest that "delayed" embryos are metabolically activated within 5 h of release from delay, perhaps because of the hormonally-induced changes in uterine proteins which occur at this time.