Survival rate after vitrification of various stages of cat embryos and blastocyst with and without artificially collapsed blastocoel cavity
Embryo vitrification is a modern technique for cryopreservation in assisted reproductive programs. From all the embryos, blastocysts are the most challenging during cryostorage due to their size, multicellular structure and the presence of blastocoelic fluid. The aim of this study was to evaluate the suitability for vitrification of various developmental stages of feline embryos and the influence of the artificial shrinkage (AS) of expanded blastocyst on post-vitrification survival rates. The AS procedure is the manual puncture of the trophectoderm allowing for the reduction of blastocoelic fluid prior to vitrification and thus preventing the ice crystal formation. The vitrified embryos were divided into groups of 2-cell, 4- to 8-cell, >8-cell, morulae, compacted and expanded blastocyst, based on morphological assessment and vitrified in groups of 1–3 embryos per Cryotop. The post-warming survival was similar regardless the embryo developmental stage prior vitrification; however, development to blastocysts was only noted in 4- to 8-cell and >8-cell vitrified embryos (13% and 27%, respectively). Following warming, the significantly more viable blastocysts were noted in vitrified compacted versus expanded blastocyst and in expanded blastocyst subjected to AS procedure versus expanded blastocyst without AS (total survival: 58.3% vs. 33.3% and 64.3% vs. 38.5%, respectively; re-expansion rate within 2 hr post-warming: 41.7 vs. 6.7% and 50% vs. 7.7%, respectively). One-fifth of vitrified expanded blastocyst showed morphological damage immediately after warming procedure, whereas no visible damage was noted in compacted blastocyst and artificially collapsed expanded ones. The obtained results suggest that the most suitable for vitrification are feline embryos containing four to 16 blastomeres and compacted blastocyst. In addition, the reduction of blastocoel cavity in expanding blastocyst by artificial collapse improved the blastocyst vitrification outcome.