The rate of development of in vitro-derived cattle embryos affects neither their survival nor their hatching rate following recovery from cryopreservation by vitrification

In pursuit of the practice of assisted bovine reproduction over many years we have often observed anecdotally that some in vitro-derived cattle embryos develop faster than others within the same cohort. Recently published research has indicated that the survival of bovine embryos following cryopreservation is influenced by their developmental rate. However, it has been reported contemporaneously that expanded blastocysts collected on day 7 and day 8 did not affect the survival of embryos after vitrification. This raises the question, therefore, as to whether the rate of an embryo’s development affects its survival post cryopreservation. Increased understanding of this physiological process may inform best practice in selection of in vitro-derived embryos for cryopreservation and for fresh embryo transfer, thereby enhancing the efficacy of in vitro fertilization technologies in cattle. The purpose of the current study was to assess the effect of the developmental rate of in vitro-derived expanded bovine blastocysts on their survival and hatching rates following vitrification. Embryos were generated in vitro from oocytes aspirated from abattoir-derived ovaries and from frozen bull sperm. A total of 112 expanded blastocysts were collected (56 each on days 6 and 7 post-insemination) and each was cryopreserved by vitrification using the standard cryotop method. After 5-10 months of storage in liquid nitrogen, embryos were warmed and cultured in vitro for 24 h in order to evaluate their re-expansion rates and for 48 h in order to examine their hatching rates. Our findings show that there were high survival rates of embryos, measured after 24 h, and no difference in hatching rates, measured after 48 h, regardless of the day they had been collected and vitrified. In conclusion, neither the survival nor the hatching rate of bovine expanded embryos was affected by the rate of embryonic development. Hence, expanded in vitro-derived blastocysts may be vitrified on either day 6 or day 7 post insemination with full expectation of good outcomes upon recovery from cryopreservation.