Abstract
The role of actin filaments and contractions in hatching was determined by time-lapse videomicrography in mouse blastocysts whose hatching ability had been suppressed by cytochalasin B (CB), an inhibitor of polymerization of actin filaments. The hatching rate of blastocysts developed from morulae in a medium containing CB at a concentration of 0.4 μg/ml (CB-treated blastocysts) was 4.3%, which was significantly lower than the 63.3% of blastocysts developed from morulae in a medium without CB (non-treated blastocysts). The rate of blastocysts with protrusion of trophectoderm cells from a small hole in the zona pellucida and that of blastocysts with a slit in the zona pellucida by enlarging the protrusion of trophectoderm cells were significantly lower in CB-treated blastocysts (66.7 and 33.3%) than in non-treated blastocysts (93.5 and 80.6%). Such a CB-treated blastocyst took a significantly longer time for the protrusion of trophectoderm cells after blastocoel formation than non-treated blastocysts. Over the span of 32 h after blastocoel formation, the number of weak contractions was similar in CB-treated and non-treated blastocysts, but the number of strong contractions (more than 20% volume reduction) was significantly larger in CB-treated blastocysts (3.43 times) than in non-treated blastocysts (1.13 times). These results suggest that actin filament-mediated movements of trophectoderm cells contribute to the hatching by facilitating the protrusion of trophectoderm cells from a small hole in the zona pellucida and also by enlarging the protrusion. In addition, it is suggested that the low ability of hatching in blastocysts is related to strong contractions with high frequency.
