The development of mammalian life begins with fertilization, the fusion of two haploid cells, oocyte and sperm, which results in the formation of the diploid zygote containing two pronuclei. For the faithful development of an organism, maintenance of its genomic integrity during early embryonic mitosis is essential as genetic abnormalities transmitted through the blastomeres can result in pregnancy failures and/or severe fetal disorders. Given its crucial importance, cell division is surprisingly error-prone at the beginning of mammalian life. The reported incidence of aneuploidy (presence of an abnormal number of chromosomes in a cell) in early human embryo exceeds 50% and is a major cause of infertility and severe congenital disorders.
The first embryonic division is of particular importance as it facilitates the union of the maternal and paternal genomes. In the past it has been thought that a single spindle combines the maternal and paternal chromosomes. However, it is recently shown that two bipolar spindles form in the zygote which independently congress the maternal and paternal chromosomes and then must be aligned to execute the first division faithfully. This intriguing spindle assembly revealed the mechanism behind the long-standing observation that parental genomes occupy separate nuclear compartments in the two cell embryo and provides a likely rationale for erroneous divisions into more than two blastomeric nuclei observed in other mammalian zygotes, including human. Indeed, preventing the alignment of the two spindles after the assembly gives rise to multi-nucleated two-cell embryos in mouse. In mouse zygotes the alignment is rather reliable if not artificially perturbed, but in zygotes of other mammalian species it seems to be much less faithful. While the discovery of the dual spindle in mammalian zygotes has provided important new basic insights into the first mitotic division, the molecular mechanisms underlying dual spindle formation are still elusive.
In this project, I established functional live-imaging analysis for the mouse zygote by combining advanced light sheet technics and molecular perturbations. With this approach, I was able to dissect the molecular mechanisms of dual spindle assembly and alignment in the first division. Indeed, I identify the major microtubules (MTs) nucleation sites and pathways responsible for dual spindle formation. Moreover, I determined the molecule essential for dual spindle alignment. Together with detailed molecular analysis of the molecules I identified, I established a new molecular model for the dual spindle formation in the mouse zygote.
