Cell cycle timing and centromere positioning in <i>Bos taurus</i> zygotes derived from <i>in vitro</i> fertilization

Centromeres are chromosomal loci essential for the correct segregation of genetic material during cell division. Defects in centromere function can lead to aneuploidy and cancer. During early embryonic development in mammals, prior to the first cell division, male and female genomes are separated in pronuclei located at the centre of the zygote. Parental chromatin clusters at the interface between the two pronuclei and this clustering step is critical to avoid aneuploidy in human and bovine zygotes. Yet, despite their essential function in chromosome segregation, the position and spatial organization of centromeres during the first cell cycle in mammals is mostly unknown. Previous studies conducted in bovine embryos derived from <i>in vitro</i> fertilization (IVF) showed that cell cycle progression impacts on the success rate of blastocyst formation. Specifically, embryos that entered earliest into S-phase or the earliest cleaving embryos were more likely to develop into blastocysts. To determine the precise timing of these events we performed a detailed characterization of key phases of the first cell cycle in bovine zygotes derived from IVF. In parallel we examined the spatial positioning of centromeres. We identify 20 h post insemination (hpi) as the timepoint when male and female pronuclei are juxtaposed and are completing S-phase. At this timepoint, we show that centromeres are positioned distal to the pronuclear interface and use super resolution microscopy to demonstrate extensive centromere clustering into chromocentres. Our results identify distinct nuclear features observed at 20 hpi, which may serve as cell cycle markers in determining successful bovine IVF.