Embryo pulsing: repeated expansion and contraction of in vivo and in vitro equine blastocysts.

Morphokinetic evaluation of embryo development has allowed the discovery of events occurring during blastulation. Here, we describe equine embryo pulsing, determined as continued expansion and contraction of both in vivo and in vitro produced blastocysts. Using time-lapse imaging, we demonstrated that pulsing starts during early blastocyst development of in vitro-produced embryos in horses. The median time for a complete contraction was 0.22h (0.08h - 2h; min-max) where embryos reduced their sizes around 12.0% (median; 2.3%-27.0%) and the median time for an expansion was 3.3h (0.75h-9.0h) where embryo re-expanded around 16.9% (3.2%-42.8%). We also found that pulsing can be observed in in vivo-produced embryos obtained from mares 6.5 days after ovulation and continues during the expansion of the blastocysts. Even though its exact mechanism remains unknown, studies in human IVF suggest that the pulsing of embryos is associated with embryo quality and implantation rates. Thus, further investigations regarding this event in equine in vitro production procedures are warranted. Additionally, the pulsing in the in vivo-produced embryos could explain the diverse morphology occasionally observed in the collected or shipped embryos. Future studies are necessary to understand the underlying mechanism of pulsing and its association with embryo quality and embryo transfer outcome.