Abstract
In cultured mouse and rat blastocysts, hatching was found to begin with protrusion of trophectoderm cells from zonae pellucidae at the expanded stage. Protrusion of the cells occurred at the mural trophectoderm and at the polar trophectoderm. After protrusion, a slit was formed in the zona pellucida in all mouse and rat blastocysts. It was also confirmed that hatching was completed in the form of either expansion or contraction in both mouse and rat blastocysts. From these results, the process of blastocyst hatching could be classified into 6 types in the mouse. Since 5 of 6 types of hatching process, classified in mouse blastocysts, were observed in rat blastocysts, it is inferred that the process of hatching differs among blastocysts, but not remarkably between the mouse and rat. The mean duration of hatching from its start to completion was 22.4 h in mouse blastocysts and 40.9 h in rat ones. When the activity of trypsin-like proteinase was histochemically examined in mouse and rat embryos, this enzyme activity was detected in those from the compacted 16-cell stage to the expanded blastocyst stage. This enzyme activity was present in both mural and polar trophectoderm cells of blastocysts, suggesting the reason for no polarity of protrusion of trophectoderm cells in mouse and rat blastocysts.
