Hatching and Distribution of Actin Filaments in Mouse Blastocysts Treated with Cytochalasin B

It is generally accepted that the hatching of mouse blastocysts is accompanied by regional dissolution of the zona pellucida by a trypsin-like proteinase synthesized in trophectoderm cells (Perona and Wassarman, 1986; Hogan et al., 1994), and trophectoderm cells protrude from the dissolved hole of the zona pellucida (Orsini and McLaren, 1967; McLaren, 1970; Niimura and Fujii, 1997). Then a slit is formed in the zona pellucida from the hole by enlargement of the protruding trophectoderm cells (Orsini and McLaren, 1967; McLaren, 1970; Niimura and Fujii, 1997). During hatching, the blastocyst repeats contractions, leading to the enlargement of the slit, and then escapes from the zona pellucida (Orsini and McLaren, 1967; McLaren, 1970; Niimura and Fujii, 1997). Generally, it is known that most animal cell motility is induced by actin filaments (Mitchison and Cramer, 1996; Lodish et al., 2008). In blastocysts also it is considered that trophectoderm cell motility is involved in hatching through the action of actin filaments (Cheon et al., 1999; Niimura and Wakasa, 2001). That is, actin filaments are distributed abundantly in trophectoderm cells of blastocysts during the hatching period compared to before and after hatching periods, and are densely distributed especially in the peripheral cytoplasm of trophectoderm cells protruding from the zona pellucida (Cheon et al., 1999). Furthermore, in mouse blastocysts treated with cytochalasin B (CB), an inhibitor of actin polymerization, the distribution of actin filaments changes (Cheon et al., 1999) and changes also occurr in contractions that increase the protrusion of trophectoderm ce l l s f rom the zona pe l luc ida and the protruded trophectoderm (Niimura and Wakasa, 2001). As a result, hatching in such the blastocysts was inhibited. From the results about the distribution of actin filaments in mouse blastocysts treated with CB, Cheon et al. (1999) suggested that dynamic polymerization of actin molecules in trophectoderm cells is essential for blastocyst hatching, and that actin filament-mediated movements of trophectoderm cells play an important role in the hatching process of mouse blastocysts. However, there have been no observations on the distribution of actin filaments in CB-treated blastocysts in the pre-hatching and post-hatching periods, in order to determine the role of actin filaments in hatching process. In the present study, the hatching rate and the distribution of actin filaments were examined in mouse