Change in the ability of bovine granulosa cells to elongate transzonal projections and their transcriptome changes during follicle development.

Granulosa cells (GCs) in secondary follicles differentiate into cumulus cells (CCs) and mural granulosa cells (MGCs) in the antral follicle. Only CCs maintain direct connections with oocytes through transzonal projections (TZPs) and support oocyte growth. Here, we examined whether granulosa cells (GCs) from secondary follicles and MGCs from early and late antral follicles were able to reconstruct complexes with TZP-free denuded oocytes (DOs) and regenerate TZPs. Furthermore, to confirm that the regenerated TZPs were functional, the development of the reconstructed complexes and oocyte growth in the complexes were evaluated. After coculture, GCs and MGCs from early antral follicles reconstructed the complexes with DOs and regenerated TZPs. Furthermore, the oocytes in the integrally reconstructed complexes grew fully and acquired meiotic competence, suggesting that the regenerated TZPs were functional. In contrast, MGCs from the late antral follicles lost their ability to elongate TZPs. As the ability to regenerate TZPs differed among cells, we analyzed the transcriptomes of GCs, CCs, and MGCs collected from follicles of different sizes. The characteristics of TZP generation coincided with the transcriptome changes in two directions: from GCs to CCs and MGCs. In conclusion, until the early antral follicle stage, bovine GCs, CCs, and MGCs have common characteristics to elongate TZPs and form antrum-like structures that support oocyte growth in vitro. Furthermore, as the follicle develops, MGCs lose the ability to elongate TZPs.