Equine ovarian tissue viability after cryopreservation and in vitro culture

Ovarian tissue cryopreservation allows the preservation of the female fertility potential for an undetermined period. The objectives of this study were to compare the efficiency of cryoprotective agents (CPAs; dimethyl sulfoxide, DMSO; ethylene glycol, EG; and propylene glycol, PROH) using slow-freezing and vitrification methods, and evaluate the viability of cryopreserved equine ovarian tissue after 7 days of culture. Fresh and cryopreserved ovarian fragments were evaluated for preantral follicle morphology, stromal cell density, EGFR, Ki-67, Bax, and Bcl-2 protein expression, and DNA fragmentation. Vitrification with EG had the highest rate of morphologically normal preantral follicles, while DMSO had the lowest (76.1 ± 6.1% and 40.9 ± 14.8%, respectively; P < 0.05). In slow-freezing, despite that DMSO had the highest percentage of morphologically normal follicles (77.7 ± 5.8%), no difference among the CPAs was observed. Fluorescence intensity of EGFR and Ki-67 was greater when vitrification with EG was used. Regardless of the cryopreservation treatment, DMSO had the highest (P < 0.05) Bax/Bcl-2 ratio; however, DNA fragmentation was similar (P > 0.05) among treatments after thawing. After in vitro culture, the percentage of normal follicles was similar (P > 0.05) between slow-freezing and vitrification methods; however, vitrification had greater (P < 0.05) stromal cell density than slow-freezing. In summary, equine ovarian tissue was successfully cryopreserved, increasing the viability of the cells in the ovarian tissue after thawing when using DMSO and EG for slow-freezing and vitrification methods, respectively. Therefore, these results are relevant for fertility preservation programs.