The aim of the study was to examine viability of cattle oocytes after cryopreservation. Oocytes after in vitro maturation (IVM) were vitrified in minimum volume on the nickel electron microscopy grids by ultra-rapid cooling technique. After warming and subsequent in vitro fertilization the presumptive zygotes were cultured to reach the stage of the blastocyst (Bl). Several devitrified oocytes were processed for electron microscopy assay. Although, embryo cleavage and Bl percentages in the vitrified group were slightly lower than in the control group (P < 0.05), the Bl total cell number (TCN), apoptosis and dead cell percentages did not differ between both groups. However, significant difference was found between day 7 (D7) and day 8 (D8) Bl in the TCN in control (108.0 vs. 90.5) and vitrified group (103.75 vs 98.14). Electron microscopy of frozen oocytes revealed slight reversible injuries in mitochondria and the smooth endoplasmic reticulum (SER), nevertheless, the development of devitrified oocytes to the Bl stage was comparable to those in fresh oocytes. In conclusion, higher proportion of slower developing Bl (D8) compared to D7 Bl may be related to the mentioned minor damages of some organelles in vitrified oocytes.
