Fig. 7. The embryonic part of the blastocyst tends to be derived from the first blastomere to divide at the two-cell stage. (A) Egg shortly after fertilisation showing the fertilisation cone (fc) with sperm tail coloured in yellow and fluorescent bead marking the sperm entry point (SEP) shown in green. Second polar body marks the animal pole. The first cleavage plane, which is marked by both the polar body and the SEP, divides the zygote into two cells (B, ‘red and blue’) that follow distinguishable fates. In the three-cell embryo (C), the blastomere that inherits the SEP (red) tends to divide first to produce cells that populate predominantly the embryonic part of the blastocyst. The first cleavage plane is reflected in the blastocyst (D) as the border between the lineages comprising the embryonic part (ICM destined to become epiblast and overlying polar trophectoderm, shown in red) and the lineages of the abembryonic part (ICM cells that are located more towards the blastocoel – thus tending to develop into primitive endoderm and mural trophectoderm, shown in blue). Blastocyst shown in a comparable orientation to the embryos in B,C to indicate its two major axes: animal-vegetal and embryonic-abembryonic. For illustration, colour has been added to DIC images of embryos to indicate lineage relationships between cells. Scale bar: 20 µm.
