Reproductive Technology

Bovine Blastocyst Development In Vitro: Timing, Sex and Viability Following Vitrification

Abstract
Selection of blastocysts based on their morphological characteristics, and the rate of development in vitro can skew the sex ratios. This study aims to determine whether an embryo's developmental rate affects its survival after vitrification, and whether male and female embryos survive vitrification differently. In vitro fertilized bovine oocytes were cultured in KSOM + 0.1% BSA for 96 h, and then into KSOM + 1% BSA (KSOM) or in sequential KSOM + 0.1% BSA for 96 h, and then into SOF + 5% FBS (KSOM-SOF). In part 1 of this study, embryos cultured in each medium that had developed into blastocysts at approximately 144, 156, or 180 h were recovered from culture, graded, and they were vitrified. After warming, blastocyst survival rates were immediately evaluated by re-expansion of the blastocoels. In the second part of this study, all blastocysts (n=191) were sexed by PCR 48 h post-warming. When cultured in KSOM medium, more 144 h blastocysts survived vitrification (68%) than blastocysts vitrified at 180 h (49%). Blastocysts derived at 156 h in KSOM-SOF survived vitrification better (87%) than blastocysts vitrified at either 144 h or 180 h, and subsequently hatched at a greater rate than those vitrified at 180 h. The overall blastocyst survival rates did not differ significantly whether embryos were cultured in KSOM or sequential KSOM-SOF. Blastocysts derived at 144 and 156 h in KSOM or KSOM-SOF were predominately male, and significantly more of them survived vitrification 48 h post-warming. However, blastocysts cultured in KSOM-SOF, and then vitrified at 180 h were predominately female. Overall, blastocysts that survived vitrification, and subsequently hatched 48 h post-warming, were male. In summary, embryos that reached the blastocyst stage earlier were predominantly males; these males have better morphology, endured vitrification, and subsequently hatched at a greater rate than did female blastocysts.

Key words: Embryo • Assisted Reproductive Technology • Early development • Fertilization • In vitro fertilization

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