graphic

Even if the first cleavage is reoriented, one blastomere becomes embryonic (red) and the other abembryonic (blue).

ZERNICKA-GOETZ/MACMILLAN

The very first division of a mammalian zygote breaks its symmetry, as shown by Berenika Plusa, Magdalena Zernicka-Goetz (University of Cambridge, Cambridge, UK), and colleagues. The early split establishes two halves that already have different developmental characteristics.Using 3D real-time images of dividing embryos, the group shows that the orientation of the first cleavage is not random. Rather, the cleavage furrow tends to form in the proximity of the sperm entry site and near the plane formed by the pronuclei (the sperm and egg genomes) just before they mix.

Zygote manipulation revealed an overriding effect of cell shape on cleavage orientation. When zygotes were slightly flattened, the cleavage plane went through the short axis of the cell, no matter where the pronuclei were positioned. Pronuclei usually align along the long axis in flattened zygotes, which may explain why past experiments suggested that the cleavage plane lay between, not parallel to, the pronuclei.

The ability to change the zygote's choice of cleavage plane allowed the authors to show that the cleavage itself predicts the blastocyst axis. Whether the cleavage was along the original or the shape-imposed axis, one resulting cell became the embryonic part of the blastocyst, while the other cell became the abembryonic part.

A more difficult problem to tackle will be determining how the two blastomeres differ. “Fate may be determined by the split of components,” says Zernicka-Goetz. But this would require that “reorienting the first cleavage resegregates other things in the egg,” she says. Alternatively, each half may be equally competent to acquire either fate, but subsequent divisions impose the separate fates.

Reference:

Plusa, B., et al.
2005
.
Nature.
434
:
391
–395.