Human ovarian cryopreservation: vitrification versus slow freezing from histology to gene expression

Background: Cryopreservation of ovarian tissue is one of the strategies offered to girls and women needing gonadotoxic treatment, as a means of preserving their fertility. There are two methods of ovarian tissue cryopreservation: slow freezing, the reference method, and vitrification, an alternative method. The aim of the present study was to evaluate which of the two is the best method for human ovarian tissue cryopreservation. Each ovary was divided into 3 groups: Fresh, Slow freezing and Vitrification. In each group a histological study to evaluate follicular density and quality; and an evaluation of 6 gene expression (CYP11A, STAR, GDF9, ZP3, CDK2, CDKN1A) were performed. Results: We observed no significant difference in follicular density within these 3 groups. Slow freezing altered the pool of primordial follicles compared to the Fresh tissue (31.8% vs 55.9%, p = 0.046, respectively). The expression of genes involved in steroidogenesis varied after cryopreservation compared to the fresh group; CYP11A was under-expressed in both freezing groups compared to the fresh group and significantly under-expressed in the slow freezing group (p = 0.01), STAR was over-expressed in the slow freezing group and significantly under-expressed in the vitrification group (p = 0.01). Regarding the expression of genes involved in cell cycle regulation, CDKN1A was significantly under-expressed in both freezing groups (slow freezing: p = 0.0008; vitrification: p = 0.03) compared to the fresh group. Conclusion: Vitrification had no effect on the histological quality of the follicles at any stage of development compared to Fresh tissue. There was no significant difference in gene expression between the two techniques.