Human oocyte cryopreservation: new perspectives regarding oocyte survival

Abstract

The success of human oocyte cryopreservation depends on morphological and biophysical factors that could influence oocyte survival after thawing. Various attempts to cryopreserve human oocytes have been performed with contrasting 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 survival after thawing were investigated. The oocytes were cryopreserved in 1,2-propanediol added with sucrose, using a slow-freezing-rapid-thawing programme. After thawing, the oocytes were inseminated by intracytoplasmic sperm injection (ICSI) and the outcomes of insemination and subsequent embryo development were also recorded. The post-thaw cryosurvival rate was not different for the oocytes cryopreserved with their cumuli partially removed mechanically (56%) when compared with those cryopreserved with their cumuli totally removed enzymatically (53%). On the contrary, a significantly higher survival rate was obtained when the oocytes were cryopreserved in the presence of a doubled sucrose concentration (0.2 mol/l) in the freezing solution and the survival rate was even higher when the sucrose concentration was tripled (0.3 mol/l) (60 versus 82% P < 0.001). Furthermore, a longer exposure time (from 10.5 to 15 min) to cryoprotectants, before lowering the temperature, significantly increased the oocyte survival rate (P < 0.005). Intracytoplasmic sperm injection produced a good fertilization rate (57%) of thawed oocytes and a high embryo cleavage rate (91%) and a satisfactory embryo morphology was observed (14 and 34% for grade I and grade II embryos respectively).

Qualitative and morphometric analysis of the ultrastructure of human oocytes cryopreserved by two alternative slow cooling protocols

“…In fact, after approximately 20 years of efforts in basic and clinical research [1,2,4,14,15,[27][28][29][30][31], oocyte cryopreservation has been described to achieve high standards of clinical efficiency [3,6,7,32]. Objective clinical evidence is emerging at a relatively slow pace, because IVF centres that routinely cryopreserve oocytes are still few and, consequently, current data are too small or dishomogeneous to draw definitive conclusions.…”

“…With the advent of improved protocols [4,34,35], novel ultrastructural evidence was generated. Comparing fresh oocytes with others frozen with a CRSC involving 0.1 mol/l sucrose as an extracellular CPA, Ghetler et al [36] found massive reduction in the number of CG as a effect of cryopreservation, concluding that stored oocytes should be microinjected rather than inseminated by standard IVF to prevent possible fertilization failure secondary to zona hardening.…”

“…Concomitantly, embryo cryopreservation has developed as the preferred form of preservation. In the last several years, though, controlled rate slow cooling (CRSC) [3][4][5] and vitrification [6,7] approaches have rapidly advanced, bringing oocyte cryopreservation into the range of treatments available for women suffering from, or at risk of, infertility. Nevertheless, the insufficiency of clinical data is still stirring controversy on the choice of the cryopres-ervation method(s).…”

Oocyte cryopreservation: is it time to remove its experimental label?

“…In the intervening years, variations to OC methods including changes in sucrose [14][15][16] and sodium [17,18] concentrations in slow freezing media, along with the first report of successful oocyte vitrification [19] and the development of novel cryotools [20][21][22][23] have combined to provide consistently improved survival and pregnancy rates with OC. In addition, although depolymerization of the oocyte's meiotic spindle is known to occur, a number of studies have now shown that the spindle reforms in the majority of oocytes after thawing [24][25][26][27][28][29].…”

Successful live birth from vitrified oocytes after 5 years of cryopreservation

“…The recent increased interest in vitrification has resulted in improved efficiency and pregnancy outcomes [2][3][4][5][6][7][8][9][10][11][12][13]. Being able to protect and prolong the reproductive capacity by way of oocyte cryopreservation potentially opens the door for many thousands of women who are at a risk of losing ovarian function from radiation, chemotherapy and surgery.Over the last decade, multiple clinics have reported IVF successes after transfer of embryos derived from frozen/ thawed oocytes [5,[14][15][16][17][18][19][20]. The authors have recently reported that oocytes from young fertile women retain their reproductive potential after 6 months of vitrification cryopreservation [20] and the pregnancy and implantation rates are comparable to that of frozen embryos.…”