Significance of the number of embryonic cells and the state of the zona pellucida for hatching of mouse blastocysts in vitro versus in vivo

We investigated the course of mouse blastocyst hatching in vitro after expe rimental modulation of the hatching process by growth hormone or by laser t reatment and compared it to embryos grown in vivo. When embryos were grown in vitro, successful hatching was dependent on blastocyst expansion and was based on a minimum number of embryonic cells. Embryos grown in the presenc e of growth hormone were more advanced in their development and hatched ear lier When an artificial opening was laser-drilled into the zona pellucida, hatching occurred at lower numbers of embryonic cells. In vivo, escape from the zona pellucida occurred earlier and independent of blastocyst expansio n. However, when we isolated in vivo-grown blastocysts with intact zonae th at had developed in vivo and then cultured them in vitro, blastocysts start ed to expand and hatched the following day when a sufficiently high number of embryonic cells was present. Our data show that successful hatching in v itro is dependent on a sufficiently high number of embryonic cells, which e nables blastocyst expansion and zona shedding. In vivo, the lower number of embryonic cells detected in zona-free blastocysts indicates that the under lying mechanism of zona escape is different, does not depend on blastocyst expansion, and presumably involves lytic factors from the uterus.