Effect of the endocrine state of blastocyst donors on the time required for initiation of trophoblast outgrowth.

Blastocysts from mice in a state of delayed implantation after ovariectomy were recovered on day 5, 7 or 9 (day 1 was the day a vaginal plug was found). Blastocysts were also recovered on day 7 from animals that had received an injection of oestradiol-17beta 8, 16 or 24 hours earlier. The blastocysts were incubated in a modified Brinster's medium to which serum had been added and the time of initiation of blastocyst outgrowth was recorded. Blastcysts from early delay grew out before those from late delay. A steady state of outgrowth time was achieved on day 7. It is therefore suggested that blastocysts for culture experiments in which the metabolic activity level is crucial should not be recovered before the steady state is attained. Oestrogen injections caused earlier outgrowths, at least after 16 h, which indicates that the earlier outgrowth soon after ovariectomy might also be caused by a residual effect of the oestrogen.