Selective vitrification of euploid oocytes markedly improves their post-warming viability and post-fertilization pregnancy generating potential, thereby opening the door to commercial egg banking

Abstract

The poor pregnancy generating potential of thawed post-fertilized, mature (M II) oocytes hitherto reported, is largely attributable to the following: a) at least 64% of oocytes are aneuploid and are thus incapable, upon fertilization, off propagating euploid (competent) embryos and, b) conventional (slow) freezing protocols commonly result in intracellular ice crystal formation and a precipitous increase in solute concentration thereby damaging the cytoskeleton of cryopreserved oocytes. We undertook a study based on the integration of the following two important recent developments that have changed this dynamic: 1) It is possible through comparative genomic hybridization (CGH) performed on PB-1s to fully karyotype oocytes and thereby selectively cryopreserve only those that are euploid and 2) Ultra-rapid oocyte cryopreservation (vitrification) avoids intracellular crystallization and rising intracellular solute concentrations. Prospective. Blastocysts obtained from frozen euploid oocytes were transferred to recipients (n = 13). Pregnancy, on-going gestation and implantation rates were evaluated. We selectively vitrified 55 euploid MII oocytes derived from 6 young oocyte donors who had undergone ovarian stimulation with gonadotropins, followed by egg retrievals. Of these 42 euploid oocytes (all of which survived) were thawed and subjected to ICSI. Thirty eight (97%) were successfully fertilized, and 22 (58%) progressed to the expanded blastocyst stage. Thereupon 2 blastocysts (mean 1.6 per ET) were transferred to the uteri of each of 13 healthy recipients aged 35Y-44Y. Eight women (61%) achieved clinical pregnancies. One (1) miscarried a euploid normally formed 16 week conceptus due to congenital cervical incompetence and seven women (7) gave birth to nine (9) normal euploid babies (there were two sets of twins). The pregnancy rate (beyond 12 weeks) was 62% (8/13) and the implantation rate per embryo transferred was 45% (10/22), which is about 4 times greater than previously reported. The results thus far obtained from this ongoing trial are indeed very encouraging and if perpetuated, could lead to expanded reproductive choices for women, herald the commercialization of egg banking for Fertility Preservation and provide more ready access to donated oocytes.