Cytoplasmic maturity of metaphase II human oocytes: Biologic importance and clinical implications for in vitro fertilization

Background: Two crucial biologic events in human fertilization are cytoplasmic maturation and nuclear maturation of the oocytes. Asynchrony between the nucleus and the cytoplasm in the maturation process may reduce fertilization outcome. This article reviews the biologic importance and clinical implications of abnormal cytoplasmic characteristics in human metaphase II (MII) oocytes (i.e., oocytes in which extrusion of the first polar body and completion of nuclear maturation has occurred) for in vitro fertilization (IVF). Methods: The three most commonly observed abnormal cytoplasmic features of MII oocytes are grainy cytoplasm, irregular gelatinous cytoplasm, and compact cumulus cells that cannot be removed by stripping and are usually accompanied by dark cytoplasm. The IVF outcome of these three types of oocytes was assessed. Results: The presence of an irregular, gelatinous cytoplasm or retention of the compact cumulus denoted oocyte incompetence with poor prognosis, which could be manifested in the fertilization rate, in abnormal fertilization, in the degeneration rate, or the early embryonic development. Conclusions: The abnormal changes in the cytoplasm of MII oocytes might be a reflection of delayed cytoplasmic maturation that is unsynchronized with nuclear maturity. Accurate characterization of MII oocytes by an agreed-upon classification (possibly by the definitions suggested above) should be done for each patient. Further studies will assist in resolving these difficult phenomena that affect IVF outcome. Possible treatment modalities include delayed insemination, cytoplasmic donation, use of different culture media with or without the addition of hormones, and use of new recombinant hormone preparations and stimulation protocols for IVF.