A New Modified Cut Standard Straw Vitrification Technique Reduces the Apoptosis of Mouse Blastocysts and Generates More Live Mouse Offspring

Effects of freezing on apoptosis and autophagy in embryos are poorly understood. This study introduces a simple and successful method (modified cut standard straw, M-CSS) for cryopreservation of mouse zygotes. Apoptosis and autophagy were investigated in cultured mouse blastocysts derived from vitrified zygotes using two vitrification containers (M-CSS vs 0.25-ml straw). The percentages of zygotes that survived and developed into blastocysts and the number of cells per blastocyst were higher in the M-CSS group than in the 0.25 ml straw group; whereas the rate of apoptosis in blastocysts was significantly lower in the M-CSS group than in the 0.25-ml straw group. The expression of the apoptosis-related gene Caspase 3 in blastocysts was higher in the 0.25-ml straw group than in the M-CSS group; however, there were no significant differences in autophagy between these two groups. Vitrified-thawed mouse zygotes were transferred into recipients. The percentage of recipients that became pregnant and the percentage of transferred zygotes that developed into live offspring were significantly lower in the 0.25-ml straw group than in the M-CSS (10.2% vs. 17.5%). In conclusion, the novel M-CSS procedure improves oocyte and embryo vitrification. The standard 0.25-ml straw vitrification procedure induces mitochondrial apoptosis in zygotes in an autophagy-independent manner.