There are no in vivo pulsations of mouse blastocysts.

An important event at the onset of the implantation process in mammals is the hatching of the blastocyst from the zona pellucida. Microcinematographic studies of in vitro preimplantation development in mice revealed the pulsatile activity of blastocysts before and during the hatching period. It is generally accepted--up to now--that the in vitro hatching is at least partially the result of repeated contractions and reexpansions of the blastocyst. The presence of pulsatile activity in vivo may confirm this hypothesis. Mouse blastocysts were obtained by a rapid flushing (5-10') from uterine horns on day 4 of pregnancy and the presence or absence of contracted blastocysts was noted. From 410 examined blastocysts only 3% were contracted as compared with the very frequent in vitro pulsations. This result suggests that in mice the in vivo pulsatile activity of blastocysts, practically, does not exist. The in vivo contractions are probably determined by the suboptimal culture conditions.