Oocyte vitrification negatively affects the Ca2+-releasing and activation potential of mouse oocytes but is potentially useful for diagnostic purposes

Abstract Research question : To what extent does vitrification affect the Ca2+-releasing and activation potential of mouse oocytes, which are commonly used to determine the oocyte activation potential of human spermatozoa? Design The effect of mouse oocyte vitrification on Ca2+ dynamics and developmental competence after oocyte activation was assessed and compared to fresh mouse oocytes. Moreover, the Ca2+ store content of the ER was determined at different time points during the vitrification-warming procedure. Finally, the Ca2+ pattern induced by cryoprotectant exposure was determined. Results After human sperm-injection into mouse oocytes, Ca2+ dynamics (MOCA) but not fertilization percentages were significantly altered (MOAT) by vitrification-warming. Ca2+ dynamics in response to SrCl2 or ionomycin were also altered by oocyte vitrification. In contrast, activation and blastocyst rates after SrCl2 exposure were not affected while activation rates after ionomycin exposure were significantly lower in vitrified-thawed oocytes; blastocyst rates were not affected. Cryoprotectant exposure was associated with a strong drop in ER Ca2+ store content. Oocytes rapidly recovered during warming and recovery in Ca2+-containing media; a threshold AUC of Ca2+ dynamics to obtain activation rates above 90% was determined. Conclusions Vitrified-warmed mouse oocytes display reduced Ca2+-releasing potential upon oocyte activation, caused by cryoprotectant exposure. With adapted classification criteria, these oocytes can be used for diagnosing oocyte activation deficiencies in patients. Evaluating the Ca2+-signalling machinery in vitrified-warmed human oocytes is required.