Orienting early cleavage

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...