Follicle Size Regulates Cytoplasmic Maturation, Lipid Content, and Embryo Cryosurvival in Bovine Oocytes Matured...

In-vitro embryo production (IVP) in cattle is limited by the suboptimal competence of oocytes matured outside the follicular environment. While maturation is often achieved, incomplete cytoplasmic maturation may impair embryo development and survival. This study evaluated the influence of follicle size on oocyte cytoplasmic maturation, lipid metabolism, cumulus cell function, and embryo cryosurvival. Oocytes were collected from small (< 6 mm), medium (6-9 mm), and large (10-20 mm) follicles and evaluated at 0, 12, and 24 h of in-vitro maturation (IVM). Organelle distribution, lipid and mitochondrial content and distribution, cortical granule distribution, and transzonal projections (TZPs) were evaluated. Gene expression was assessed in surrounding cumulus cells, and embryo development and cryotolerance were evaluated following fertilization and culture. While organelle patterns did not differ among follicle sizes for mitochondria, lipids, or cortical granules, oocytes from large follicles had lower lipid accumulation by 24 h IVM, and fewer TZPs throughout maturation. Gene expression in cumulus cells from large follicles also had a greater abundance of transcripts involved in steroidogenesis, growth factor signaling, and glucose metabolism midway through maturation. Although cleavage and blastocyst development rates were similar across groups, embryos from large follicle oocytes had higher post-thaw survival and lower lipid content than those from small follicles. These findings demonstrate that follicle size affects key metabolic and structural aspects of oocyte maturation, ultimately influencing embryo quality. Selecting oocytes from larger follicles may improve embryo cryosurvival by promoting more complete cytoplasmic maturation and more efficient lipid regulation during IVM.