RESUMO
BACKGROUND: Timing of the first zygotic cleavage is an accurate predictor of embryo quality. Embryos that cleaved early (EC) have been shown to exhibit higher develop- mental viability compared to those that cleaved at a later period (LC). However, the vi- ability of EC embryos in comparison to LC embryos after vitrification is unknown. The present study aims to investigate the post-vitrification developmental viability of murine EC versus LC embryos. MATERIALS AND METHODS: In this experimental study, female ICR mice (6-8 weeks old) were superovulated and cohabited with fertile males for 24 hours. Afterwards, their ovi- ducts were excised and embryos harvested. Embryos at the 2-cell stage were catego- rized as EC embryos, while zygotes with two pronuclei were categorized as LC embryos. Embryos were cultured in M16 medium supplemented with 3% bovine serum albumin (BSA) in a humidified 5% CO2atmosphere. Control embryos were cultured until the blastocyst stage without vitrification. Experimental embryos at the 2-cell stage were vitri- fied for one hour using 40% v/v ethylene glycol, 18% w/v Ficoll-70 and 0.5 M sucrose as the cryoprotectant. We recorded the numbers of surviving embryos from the control and experimental groups and their development until the blastocyst stage. Results were analyzed using the chi-square test. RESULTS: A significantly higher proportion of EC embryos (96.7%) from the control group developed to the blastocyst stage compared with LC embryos (57.5%, P<0.0001). Similarly, in the experimental group, a significantly higher percentage of vitrified EC embryos (69.4%) reached the blastocyst stage compared to vitrified LC embryos (27.1%, P<0.0001). CONCLUSION: Vitrified EC embryos are more vitrification tolerant than LC embryos. Prese- lection of EC embryos may be used as a tool for selection of embryos that exhibit higher developmental competence after vitrification.