Fig. 5. Several possible models depicting the relationship between the first cleavage plane and the boundary between embryonic and abembryonic parts of the blastocyst. (A) Model by Piotrowska and Zernicka-Goetz (Piotrowska and Zernicka-Goetz, 2001). The left-hand diagram indicates the position of the first cleavage that divides the zygote into two (orange and blue) cells. The first cleavage plane (outlined in black) is set in relation to the animal pole (marked by the polar body shown in green) and the SEP (the sperm head is shown in yellow). Depending on whether cleavage passes on one or other side of these structures, its actual position with respect to the animal pole might vary by several degrees (arrow, plane outlined in green). The two-cell stage blastomere that inherits the SEP tends to divide first to produce cells that tend to populate the embryonic part of the blastocyst (right-hand diagram). The first cleavage plane is reflected in the blastocyst as the border (black disc) between the embryonic region (Em (blue); polar trophectoderm and ICM destined to become epiblast) and the abembryonic region (Ab (orange); mural trophectoderm and ICM that will contribute cells to primitive endoderm). Blastocoel in grey. The precise position of this border may reflect variations in the first cleavage plane (arrow leading to green disc). (B) Variation on the model in A in which factors influencing both the embryonic-abembryonic axis and the first cleavage plane are inter-dependent. In this model, the first cleavage plane of the zygote, and hence the clonal border of the blastocyst stage (outlined in green), is set by the relative positions of the polar body and SEP. The morphology of the blastocyst defined by a plane separating the embryonic and abembryonic parts (outlined in black) is depicted as being independently set by factor(s) (white arc) present in the animal pole cytoplasm. (C) Variation on the model in A in which the animal pole marked by the polar body is the primary determinant of the cleavage plane of the zygote or clonal border of the blastocyst (outlined in green). The displacement of the plane separating embryonic and abembryonic parts (outlined in black) is depicted as being influenced by the variation in the position of sperm entry (arrows).