Microfilament‐Myosin II Regulates the Differentiation of Multinucleated Cysts into Oocytes and Influences Oocyte Developmental Potential in Mice

Abstract Primary oocyte differentiation in females is a complex and selective process; however, its regulatory mechanisms remain poorly understood. In this study, multinucleated cysts are identified as precursors for oocyte differentiation. Disruption of microfilament dynamics disrupts the differentiation of multinucleated cysts into oocytes, resulting in reduced oocyte volume, polarity defects, Balbiani body (B‐body) absence, and double‐nucleated oocytes. Proteomics analysis reveals that microfilament depolymerization alters the myosin II isoform expression and decreases myosin II activity. Subcellular localization analysis of myosin II in multinucleated cysts demonstrates that myosin IIA co‐localized with microfilaments in the cortex and cytoplasm; myosin IIB is associated with the Golgi apparatus; and myosin IIC is distributed diffusely in the cytoplasm. Pharmacological inhibition of myosin II activity or specific knockdown of myosin II isoforms leads to oocyte differentiation abnormalities, including reduced oocyte volume, failed B‐body formation, and double‐nucleus phenotypes. Compromised differentiation quality of oocytes has long‐term consequences, with defective oocytes showing impaired developmental progression, elevated apoptosis, and diminished meiotic competence. These defects are evidenced by reduced rates of germinal vesicle breakdown (GVBD) and first polar body (PB1) excretion, mitochondrial dysfunction, reduced Golgi and endoplasmic reticulum components, and increased spindle abnormalities. In conclusion, this study identifies multinucleated cysts as precursors for oocyte differentiation, highlighting the importance of microfilament myosin II in oocyte differentiation and its long‐term influence on oocyte differentiation quality.