Morphological organization of the mouse preimplantation embryo.

During the 4 days after fertilization, the mouse embryo proceeds in its development within the zona pellucida and reaches the blastocyst stage, when a newly formed embryonic-abembryonic (E-Ab) axis is settled before implantation. The previous findings suggested that the shape of the zona pellucida, which may function as an environmental cue for the embryo proper, is a major factor affecting the specification of the blastocyst axis orientation. The characteristics of the cells were not dependent on the shape of the zona pellucida. It is suggested that the division order of cells is under certain rules, and the differentiation of cells is dependent on the relative position of blastomeres in later stages.