Nuclear Tracking During Early Embryonic Divisions

Positioning of the largest organelle, the nucleus, provides important cues for the geometry of cell divisions and cell fate determination. A primary function of nuclear positioning is to determine the spindle position and cleavage plane localization during cell division. Thus, centrally located nuclei lead to symmetrical divisions typical of mitosis, and off-center spindles culminate in symmetry-breaking as in mammalian oocytes. More recently, nuclear positioning has also been implicated in regulating mechanotransduction and gene expression in mouse oocytes. In human embryos, equal blastomere size arising from symmetrical cleavage-stage divisions is associated with favorable developmental outcomes, whereas off-center nuclear positioning affects division symmetry that can derail development. Therefore, investigating the mechanisms involved in nuclear positioning is critical for understanding embryo biology and the basis for clinical infertility. Here, we describe a method involving confocal imaging for performing nuclear tracking during early embryonic divisions in mouse embryos.