Symmetric cell division of the mouse zygote requires an actin network.

Positioning of the cleavage plane is regulated to ensure proper animal development. Most animal cells rely on the astral microtubules to position the mitotic spindle, which in turn specifies the cleavage plane. The mouse zygote lacks discernible astral microtubules but still divides symmetrically. Here, we demonstrate a cloud-like accumulation of F-actin surrounds the spindle in zygotes and when this actin network is disassembled, the spindle assumes an off-center position, and the resulting zygote divides asymmetrically into two unequal size blastomeres. Interestingly, when the spindle is micromanipulated to the subcortical region, the zygote without the actin network is unable to reposition the spindle and cleavage plane at the cell center. This study reveals that an actin network maintains the central spindle position in anastral mitosis, and ensures the first embryonic mitosis is symmetrical. © 2012 Wiley Periodicals, Inc.