Assessing equine embryo developmental competency by time-lapse image analysis

The timing of early mitotic events during preimplantation embryo development is important for subsequent embryogenesis in many mammalian species, including mouse and human, but, to date, no study has closely examined mitotic timing in equine embryos from oocytes obtained by ovum pick-up. Here, cumulus-oocyte complexes were collected by transvaginal follicular aspiration, matured in vitro and fertilised via intracytoplasmic sperm injection. Each fertilised oocyte was cultured up to the blastocyst stage and monitored by time-lapse imaging for the measurement of cell cycle intervals and identification of morphological criteria indicative of developmental potential. Of the 56 fertilised oocytes, 35 initiated mitosis and 11 progressed to the blastocyst stage. Analysis of the first three mitotic divisions in embryos that formed blastocysts determined that typical blastocyst timing (median ± IQR) is 30.0 ± 17.5 min, 8.8 ± 1.7 h and 0.6 ± 1.4 h respectively. Frequent cellular fragmentation, multipolar divisions and blastomere exclusion suggested that equine embryos likely contend with a high incidence of chromosomal missegregation. Indeed, chromosome-containing micronuclei and multinuclei with extensive DNA damage were observed throughout preimplantation embryogenesis. This indicates that time-lapse image analysis may be used as a non-invasive method to assess equine embryo quality in future studies.