First cleavage is a manifestation of the geometry of the unfertilized oocyte: implications for monozygotic twinning in mice.

A long-standing question in mammalian embryology is whether regional differences of oocyte composition matter for the properties of blastomeres receiving those regions after fertilization. A hitherto untested hypothesis is that allocation depends on the orientation of 1st cleavage. However, the orientation is influenced by the site of sperm entry in the oocyte, which can be almost anywhere on the membrane of oocytes when these are inseminated. Therefore, we harnessed the intracytoplasmic sperm injection to impose the site of fertilization in three specific ooplasmic regions (animal pole, vegetal pole, equator) of mouse oocytes. Notwithstanding this categorical distinction, the sister blastomeres of resultant 2-cell embryos differed from each other in the same way, as measured by gene expression and twin blastocysts formation following bisection. We reasoned that either the oocyte territories did not matter, or their effect was obscured by other factors. To shed light on these possibilities, we immobilized the oocytes on the micromanipulation stage during sperm injection and for 24 h thereafter. Imaging revealed that the orientation of 1st cleavage, instead of depending on the fertilization site, depended on the shorter diameter of the unfertilized oocyte. This even led to the segregation of animal and vegetal hemispheres in the blastomeres of 2-cell embryos. We conclude that 1stcleavage is a manifestation of the intrinsic organization of the oocyte prior to fertilization, and that oocyte geometry factors in to the patterning of mouse embryos more so than fertilization topology.