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. Overall design: The samples are monozygotic twin embryos of the mouse, at Theiler stages 2 and 5, generated as follows. Fertilized oocytes were produced by intracytoplasmic sperm injection (ICSI) of CD1 sperm into B6C3F1 oocytes, ensuring that the sperm head was deposited in the oocyte opposite the meiotic spindle (contralateral ICSI), or next to the meiotic spindle (isolateral ICSI) or half way in between (equatorial ICSI). Fertilization was followed by pool culture in KSOM medium containing aminoacids for one cell cycle. At the 2-cell stage, the sibling blastomeres were separated from each other (Theiler stage 2) and cultured individually to the blastocyst stage (Theiler stage 5), thereby producing twin blastocysts, as described (PMID 28811525). The samples used for RNA sequencing were the individual blastomeres (Theiler stage 2) or the individual blastocysts (Theiler stage 5), lysed keeping track of the type of ICSI (contralateral, Co; isolateral, Iso; equatorial, Equat), of the precursor embryo (e.g.embryo no.1, no.2, no.3, ..., no.132), and of the original pair association (e.g. blastomere 'a' and 'b'). Accordingly, the sample denomination "ICSI_Co_1a, blastomere" means that the blastomere 1a was obtained from an oocyte (no.1) that was fertilized opposite the meiotic spincle (contralateral), and that the blastomere was one ('a') of the two from the same 2-cell embryo. The sample denomination 'ICSI_Co_intact_76, blastocyst' means that blastocyst 76 was obtained from an oocyte (no.76) that was fertilized opposite the meiotic spindle (contralateral) and that the embryo was not bisected (intact). The sample denomination 'ICSI_Eq_twin_112a, blastocyst' means that the twin blastocyst 112a was obtained from an oocyte (no.112) that was fertilized at the equator and that was bisected at the 2-cell stage, so that twin blastocysts 112a and 112b arose from the in vitro culture of the sibling blastomeres.