Physiological oocyte maturation via intrafollicular transfer improves metabolic and molecular quality of bovine...

In vitro maturation (IVM) has been widely used in bovine embryo production but is often associated with increased lipid accumulation and reduced cryotolerance. This study evaluated the intrafollicular transfer of immature oocytes (IFIOT) as an alternative oocyte maturation strategy, comparing it with conventional IVM in terms of embryo development, lipid metabolism, gene expression, cryosurvival, and DNA methylation. After maturation, oocytes were fertilized and cultured in vitro. Embryos were assessed for cleavage and blastocyst rates, lipid and mitochondrial content, total cell number, and survival after cryopreservation. Expanded blastocysts at Day 7 were transferred to synchronized recipients, and Day 14 embryos were collected for gene expression and methylation analyses. Cleavage and blastocyst rates were similar between groups (P > 0.05), but lipid content was significantly higher in IVM embryos (P < 0.05). Expression of PPARγ, a key transcription factor regulating lipid metabolism, was significantly higher in IVM-derived embryos (P < 0.0001), suggesting a more unbalanced metabolic profile compared to IFIOT. Mitochondrial activity and cell number did not differ. Overall cryosurvival was comparable; however, Day 8 IVM embryos showed reduced post-thaw survival relative to IFIOT (P < 0.05). DNA methylation levels in Satellite I and α-Satellite regions remained low in both groups, with no significant differences. These findings indicate that IFIOT supports oocyte maturation under more physiological conditions, leading to embryos with improved metabolic balance, reduced lipid accumulation, and enhanced cryotolerance. IFIOT may represent a valuable tool to optimize oocyte quality and increase the robustness of bovine in vitro embryo production systems.