Assessment of ovarian tissue and follicular integrity after cryopreservation via slow freezing or vitrification followed by in vitro culture

Objective

To evaluate ovarian tissue and follicle integrity before and after slow freezing or vitrification and postthawing in vitro culture.

Design

A laboratory study using bovine ovarian cortical tissue.

Setting

Academic laboratory.

Animals

Ovaries from healthy cattle.

Interventions

Bovine ovarian cortical tissue was subjected to either slow freezing or vitrification and subsequent in vitro culture. Tissue and follicle integrity were assessed before and after cryopreservation and culture.

Main Outcome Measures

Hematoxylin and eosin staining was used to assess follicle stages, morphology, and stromal cell density. Terminal deoxynucleotidyl transferase dUTP nick end labeling staining was used to examine apoptosis, and Masson’s trichrome staining was used to evaluate collagen content in the stromal environment. Immunofluorescent labeling was used to localize and quantify connexin 37 (CX37) and Ki67 expression.

Results

Regardless of previous cryopreservation, ovarian tissue culture resulted in a decreased percentage of primordial follicles and an increased percentage of primary follicles compared with fresh tissue, indicating that follicle activation was not negatively affected by cryopreservation. However, both culture and cryopreservation followed by culture decreased the percentage of normal preantral follicles compared with fresh tissue that had not been cultured. Culture and/or cryopreservation did not impact stromal cell number, but there was increased cell apoptosis in tissue that was cultured after vitrification compared with tissue that was not cultured. Tissue culture, regardless of cryopreservation, resulted in decreased collagen deposition. There were fewer follicles expressing CX37 in vitrified and thawed tissue compared with all other treatments. Cryopreservation and/or culture of ovarian tissue did not change the percentage of follicles that contained Ki67-positive granulosa cells or the percentage of Ki67-positive granulosa cells within those follicles.

Conclusion

Based on these data, we conclude that tissue cryopreservation followed by culture does not affect follicle activation and growth, but it decreases the proportion of viable follicles within the tissue. Slow freezing was superior to vitrification as indicated by a higher proportion of follicles with normal morphology, lower stromal cell apoptosis, and maintenance of CX37 expression postthawing and after culture.