<scp>TFAM</scp> ‐Mediated <scp>mtDNA</scp> Replication is Essential for Developmental Competence of In Vitro Grown Oocytes

Purpose: Mitochondria are essential for oocyte maturation and early embryonic development, supplying ATP and maintaining mitochondrial DNA (mtDNA) integrity. During oogenesis, mtDNA undergoes dramatic amplification, but the mechanisms and functional significance of this process remain unclear. The purpose of this study was to elucidate the role of mitochondrial transcription factor A (TFAM) in mouse oocytes using an in vitro growth (IVG) system. Methods: was microinjected into secondary follicles, which were then cultured for 12 days under IVG conditions. Following culture, oocyte growth, mtDNA content, mitochondrial membrane potential, and developmental competence after in vitro fertilization (IVF) were evaluated. Results: mtDNA copy number increased nonlinearly during oocyte growth, with a pronounced rise at the secondary follicle stage accompanied by TFAM upregulation. TFAM knockdown reduced mtDNA copy number and mitochondrial function without affecting oocyte size or meiotic maturation, but significantly decreased blastocyst formation and total cell numbers per blastocyst. Conclusions: TFAM-mediated mtDNA replication is crucial for mitochondrial function and developmental competence of IVG-derived oocytes, underscoring its importance in early embryonic development.