Vitrification of mouse oocyte versus slow freezing: evaluation of ultrastructure and developmental potential

Development of oocyte cryopreservation techniques have lead to major breakthroughs in human assisted reproductive technology, but the effect of cryopreserving oocytes is not ideal. This study compared the ultrastructure and developmental potential between the two approaches applied to mouse oocytes. In the control group, oocytes were cultured immediately, while in the vitrification and slow freezing groups, the oocytes were cultured after vitrification-warming and slow freezing-thawing procedures. The morphology and angle of spindle deviation with the first polar body were estimated by Polscope. Moreover, the measurement of the thickness and density of zona pellucida was also carried out by Polscope. The image of the morphology of oocyte was obtained by SEM, while the morphological characteristics of mitochondria and perivitelline space were observed by TEM. The slow freezing approach reduced the number of ICSI embryos that developed to the 2-cell or blastocyst stage as compared to the vitrification-thawing group. A higher quality of oocytes was observed by the vitrification-thawing protocol as compared to the slow freezing-thawing protocol. The angle of spindle deviation concerning the first polar body was in a better grade of vitrification-thawing group as compared to the slow freezing-thawing group. The developmental potential was superior in the vitrification-thawing group that was evaluated by the percentage of 2-cell and blastocyst after ICSI. These preliminary results indicated that cryopreservation significantly affected the conservation of oocytes. Oocyte vitrification by the cryotop method seems to be better than slow freezing based on the spindle deviation angles, oocyte morphology, and developmental potential.