Maturation-inhibiting activity in growing mouse oocytes

Growing mouse oocytes incompetent to resume meiosis were fused with fully grown immature mouse oocytes and cultured for 20-24 h. In giant cells that developed, two intact germinal vesicles remained well conserved in all cases. We propose that the cytoplasm of growing oocytes possesses a maturation-inhibiting activity which is able to arrest, after fusion, nuclear maturation in fully grown oocytes competent to mature spontaneously.