A coeloblastula is usually formed with the blastomeres comprising a single layered epithelium enclosing a large blastocoele (Nyholm, 1949; Mergner, 1971; Campbell, 1974; Spaulding, 1974; Fautin et al., 1989).
A major factor that affects the survival rate of blastocyst is the fluid-filled cavity called blastocoele. As expected, the formation of intracellular ice crystals is directly proportional to the volume of blastocoele.
Furthermore, it was reported that in mice, during the in vitro culture of embryos, EGF promoted the blastocoele formation (Wood and Kaye, 1989), and it also increased the blastocyst development rate of bovine and porcine embryos (Harper and Brackett, 1993; Wei et al., 2001).
As fluids accumulate, the inner cell mass is displaced to a polar region of the embryo, resulting in formation of a fluid-filled cavity called the blastocoele. The cells of the inner cell mass move as a unit because they are joined together by gap junctions.
The increase in oxygen consumption coincident with blastocyst formation is thought to reflect an increased metabolic demand generated by the pumping of sodium and potassium ions that drives the enlargement of the blastocoele (9).