Vitrified sheep isolated secondary follicles are able to grow and form antrum after a short period of in vitro culture

The risk of reintroducing malignant cells after ovarian graft into patients following post-cancer treatment is an obstacle for clinical applications (autotransplantation). In this context, in vitro follicle culture would be an alternative to transplantation in order to minimize such risks. Therefore, the aim of this study was to compare the development of secondary follicles after vitrification in isolated form (without stroma) with vitrification in in situ form (within fragments of ovarian tissue). Follicles were first isolated from ovarian fragments from mixed-breed ewes and then vitrified; these comprised the Follicle-Vitrification group (Follicle-Vit), or fragments of ovarian tissue were first vitrified, followed by isolation of the follicles, resulting in the Tissue-Vitrification group (Tissue-Vit). Control and vitrified groups were submitted to in vitro culture (6 days) and follicular morphology, viability, antrum formation, follicle and oocyte diameter, growth rate, ultrastructural characteristics and cell proliferation were evaluated. The percentages of morphologically normal follicles and antrum formation were similar among groups. Follicular viability and oocyte diameter were similar between Follicle-Vit and Tissue-Vit. The follicular diameter and growth rate of Follicle-Vit were similar to the Control, while those of Tissue-Vit were significantly lower compared to the Control. Both vitrified groups had an augmented rate of granulosa cellular proliferation compared to Control. Secondary follicles can be successfully vitrified before or after isolation from the ovarian tissue without impairing their ability to survive and grow during in vitro culture.