Human oocyte cryopreservation: new perspectives regarding oocyte survival

The success of human oocyte cryopreservation depends on morphological and b iophysical factors that could influence oocyte survival after thawing, Vari ous attempts to cryopreserve human oocytes have been performed with contras ting results, Therefore the effect of some factors, such as the presence or absence of the cumulus oophorus, the sucrose concentration in the freezing solution and the exposure time to cryoprotectants, on human oocyte surviva l after thawing were investigated, The oocytes were cryopreserved in 1,2-pr opanediol added with sucrose, using a slow-freezing-rapid-thawing programme . After thawing, the oocytes were inseminated by intracytoplasmic sperm inj ection (ICSI) and the outcomes of insemination and subsequent embryo develo pment were also recorded, The post-thaw cryosurvival rate was not different for the oocytes cryopreserved with their cumuli partially removed mechanic ally (56%) when compared with those cryopreserved with their cumuli totally removed enzymatically (53%), On the contrary, a significantly higher survi val rate was obtained when the oocytes were cryopreserved in the presence o f a doubled sucrose concentration (0.2 mol/l) in the freezing solution and the survival rate was even higher when the sucrose concentration was triple d (0.3 mol/l) (60 versus 82% P < 0.001). Furthermore, a longer exposure tim e (from 10.5 to 15 min) to cryoprotectants, before lowering the temperature , significantly increased the oocyte survival rate (P < 0.005). Intracytopl asmic sperm injection produced a good fertilization rate (57%) of thawed oo cytes and a high embryo cleavage rate (91%) and a satisfactory embryo morph ology was observed (14 and 34% for grade I and grade II embryos respectivel y).