Hatching and distribution of actin filaments in mouse blastocysts treated with cytochalasimn B

The hatching rate and the distribution of actin filaments were observed in mouse blastocysts whose ability of actin polymerization had been suppressed by cytochalasin B (CB), in order to examine the role of actin filaments in blastocyst hatching. The hatching rate of blastocysts developed from morulae in a medium containing 0.4 microg/ml CB was 9.7%, which was significantly lower than the 78.1% of control blastocysts developed from morulae in a medium without CB. The fluorescence showing the presence of actin filaments was observed in the cytoplasm of trophectoderm and inner-cell-mass cells in blastocysts, and was especially strong in the peripheral cytoplasm where two trophectoderm cells adhering to each other. Also actin fluorescence was much brighter in protruded trophectoderm cells at the region of small hole or slit in the zona pellucida of hatching blastocysts. Such the distribution of actin filaments was similar in CB-treated and control blastocysts in each hatching period. On the other hand, the rate of CB-treated blastocysts in which most trophectoderm cells possess the fluorescence of actin filaments did not differ from that of control blastocysts in the post-hatching period, whereas such the rates of CB-treated blastocysts in the pre-hatching and hatching periods (34.1 and 46.5%) were significantly lower than the 95.0 and 82.9% of control blastocysts, respectively. The CB-treated blastocysts in the pre-hatching and hatching periods had many trophectoderm cells devoid of the fluorescence of actin filaments. From these findings, it was suggested that the polymerization of actin is essential for blastocyst hatching, and 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 for slit formation in the zona pellucida.