Characterization of isolated bovine preantral follicles based on morphology, diameter and cell number

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.

Method of Isolation and In Vitro Culture of Primordial Follicles in Bovine Animal Model

The mammalian ovary is a substantial source of oocytes arranged into follicles at various stages of folliculogenesis, from the primordial to the ovulatory ones. Primordial follicles constitute the most abundant source of gametes inside the mammalian ovary at any given time.The isolation of a high number of primordial follicles, together with the development of protocols for in vitro follicle growth, would provide a powerful tool to fully exploit the female reproductive potential and boost the rescue and restoration of fertility in assisted reproduction technologies in human medicine, animal breeding, and preservation of threatened species. However, the most significant limitation is the lack of efficient methods for isolating a healthy and homogeneous population of viable primordial follicles suitable for in vitro culture. Here, we provide a fast and high-yield strategy for the mechanical isolation of primordial follicles from limited portions of the ovarian cortex in the bovine animal model.

Why Is It So Difficult To Have Competent Oocytes from In vitro Cultured Preantral Follicles?

The developmental competence of oocytes is acquired gradually during follicular development, mainly through oocyte accumulation of RNA molecules and proteins that will be used during fertilization and early embryonic development. Several attempts to develop in vitro culture systems to support preantral follicle development up to maturation are reported in the literature, but oocyte competence has not yet been achieved in human and domestic animals. The difficulties to have fertilizable oocytes are related to thousands of mRNAs and proteins that need to be synthesized, long-term duration of follicular development, size of preovulatory follicles, composition of in vitro culture medium, and the need of multi-step culture systems. The development of a culture system that maintains bidirectional communication between the oocyte and granulosa cells and that meets the metabolic demands of each stage of follicle growth is the key to sustain an extended culture period. This review discusses the physiological and molecular mechanisms that determine acquisition of oocyte competence in vitro, like oocyte transcriptional activity, follicle and oocyte sizes, and length and regulation of follicular development in murine, human, and domestic animal species. The state of art of in vitro follicular development and the challenges to have complete follicular development in vitro are also highlighted.