The maternal‐to‐zygotic transition: An asynchronous split

The early cell cycles of preimplantation embryo development are unique in the scheme of mitotic cell proliferation as cell division is not coupled to cell growth, leading to a halving of blastomere volume with each cleavage event. Among the early mouse embryonic divisions, the fi rst two are particularly different, lasting almost twice as long as subsequent divisions. The third cell cycle is marked by the transition of a four‐cell embryo into an eight‐cell embryo, and represents the fi rst complete cell cycle occurring after activation of the zygotic genome. The G2/M phase of the third cell cycle is highly variable, lasting between 2–5 hours, and heterogeneity between blastomeres within the same embryo may occur as a part of normal development. The embryo in this image is actively undergoing cleavage from the four‐ to the eight‐cell stage, and blastomeres are captured in multiple phases of the cell cycle, as visualized by chromatin structure (DNA, blue) and microtubule staining (α‐tubulin, green). Two blastomeres sit in interphase with decondensed chromatin masses and a mesh‐like microtubule network, while the remaining blastomeres are actively undergoing mitosis. Of the latter, one is in metaphase, one in early anaphase, and the last in late anaphase. All together, the diversity in cell cycle stages reveals the inherit asynchrony existent within individual blastomeres of a cleavage stage embryo. Mol. Reprod. Dev. 80: 1–1, 2012. © 2012 Wiley Periodicals, Inc.