The dominant follicle: the final frontier in bovine oocyte development

The acquisition of oocyte competence in cattle, encompassing both cytoplasmic and nuclear maturation, is essential for successful fertilization and embryonic development. This competence is progressively achieved during the latter stages of the oocyte growth phase and completed within the dominant follicle (DF). The unique hormonal and immunological environment of the DF during oestrous supports oocyte "capacitation," a process involving organelle reorganization, mRNA storage and meiosis resumption, which fully prepares the oocyte for fertilization. These changes differentiate oocytes from the DF from those of subordinate follicles, explaining why only oocytes from the DF mature and ovulate successfully. Despite advances in assisted reproductive technologies like in vitro maturation (IVM) and in vitro fertilization (IVF), developmental outcomes remain inferior compared to in vivo matured oocytes, largely due to incomplete or altered oocyte maturation in vitro. Blastocyst rates after IVM/IVF are substantially lower (~35%) than those from in vivo matured oocytes (58-78%). The heterogeneity of oocytes retrieved from antral follicles and the lack of exposure to the natural follicular environment during IVM are key factors limiting developmental competence. Here we describe the molecular changes in bovine oocytes from DFs, collected at 24 and 2 h before ovulation without ovarian stimulation, using single-cell RNA sequencing and bisulfite sequencing to assess gene expression and DNA methylation dynamics. Results revealed significant shifts in transcripts related to oxidative phosphorylation, highlighting the crucial role of energy metabolism during oocyte capacitation. DNA methylation changes were subtle but indicated a more dynamic and less stable epigenome in fully-grown oocytes than previously assumed. Overall, understanding the gene expression and epigenetic landscape during oocyte maturation in the DF offers valuable insights into improving oocyte quality and ART outcomes in cattle. Optimizing the maturation environment to better mimic natural follicular conditions could enhance reproductive efficiency in bovine production systems.