The Mechanism of Mammalian Embryo Hatching

Successful implantation in mammals requires hatching of the blastocyst from the zona pellucida. But the mechanism of mammalian embryo hatching is still not clear: Some data show that successful hatching in vitro is dependent on a sufficiently high number of embryonic cells, which enables blastocyst expansion and zona shedding. In vivo, the lower number of embryonic cells detected in zona free blastocysts indicates that the underlying mechanism of zona escape is different, it does not depend on blastocyst expansion, and presumably involves lytic factors from the uterus. Results of experiments suggest that a proteinase called strypsin is directly involved in hatching of isolated mouse blastocysts in vitro . Strypsin is a trypsin like proteinase, and it is present in mouse blastocyst and exhibits certain properties characteristic of membrane associated enzymes.These results suggest that not only tension but also lytic enzymes are required to rupture from zona pellucida.