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Selective Vitrification of Euploid Oocytes Improves Survival, Fertilization and Pregnancy-Generating Potential

Wednesday, October 1, 2008

Keywords: CGH, implantation, oocyte, pregnancy, vitrification

by Geoffrey Sher, M.D., L. Keskintepe, T. Mukaida, M. Keskintepe, M. Ginsburg, Y. Agca, G. Maassarani, A. Bayrak

Enthusiasm for oocyte cryopreservation has been limited by poor pregnancy rates per thawed metaphase II (MII) oocytes (<4%) and low implantation rates per embryos. The reasons relate to technical limitations in the freezing process, and the fact that <40% of oocytes are euploid and unable to produce ‘competent’ embryos. Comparative genomic hybridization was performed on the first polar body (PB-1) of 323 MII oocytes retrieved from 16 donors. Of these, 111 were euploid, and were vitrified. Seventy-five of 78 vitrified oocytes (96%) survived warming and were fertilized using intracytoplasmic sperm injection. Thirty-one (41%) subsequently developed into expanded blastocysts, of which no more than two were subsequently transferred per uterus to 16 out of 19 prospective embryo recipients. Twelve of 19 (63%) recipients produced 17 healthy babies (eight singletons, three twins, and one set of triplets). One twin pregnancy miscarried in the late first trimester. The birth rate per transfer of a maximum of two blastocysts to 16 recipients was 75%. The implantation rate per vitrified euploid oocyte was 27%.

This study showed a six-fold improvement in pregnancy rate per cryopreserved oocyte over previous reports and a marked improvement in implantation rate. If independently validated, this approach could open the door to commercial egg cryobanking, significantly expanding women’s reproductive choices.

In 1986, Chen reported the first pregnancy resulting from the transfer of embryos derived from previously cryopreserved oocytes (Chen, 1986). Since then, fewer than 350 births have been recorded in the scientific literature, although recent reports have been more encouraging (Isachenko et al., 2005; Kuwayama et al., 2005).
The success of human oocyte freezing is a function of the method of cryopreservation used and the ‘quality’ of the eggs selected. In the last few years, ultra-rapid embryo/oocyte vitrification has all but supplanted the conventional methods of cryopreservation. This has resulted in a significant improvement in post-thaw/warming oocyte survival and viability, as well as subsequent fertilization and pregnancy potential following embryo transfer (Kulleshova and Lopata, 2002; Kuwayama, 2007). Despite such enhancements, the highest reported pregnancy rate achieved per cryopreserved oocyte after fertilization and embryo transfer has been approximately 4% (Coticchio et al., 2007), thereby rendering the widespread introduction and/or commercialization of oocyte cryobanking both impractical and inadvisable.

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