Risk of premature chromatid separation is increased by poor cumulus cell layers and inappropriate culture media for in vitro maturation of mouse oocytes

Recently, the quality of in vitro matured (IVM) oocytes has been improved by numerous studies using animal models. However, the implantation rates of IVM oocytes remain lower than those of in vitro fertilization (IVF). Chromosome abnormalities, which mainly occur during the first meiotic division, are often observed in IVM oocytes. To clarify how the properties of the cumulus cell layers relate to chromosome abnormalities of oocytes during IVM, we investigated the occurrence of aneuploidy and developmental competence using two different media in a mouse model. Maturation rates were significantly lower in denuded oocytes at oocyte collection in both types of maturation media as compared with those of oocytes mechanically denuded just after collection of cumulus-oocyte complex. Fertilization rates and blastocyst rates were lower in Waymouth's MB752/1 medium than in HTF medium. The occurrence of premature chromatid separation (PCS) remarkably increased in Waymouth's MB752/1 medium compared to HTF medium. This demonstrates that denuded oocytes at oocyte collection have low ability to progress to nuclear maturation under IVM conditions. However, the fertilization ability and developmental competence to the blastocyst stage of denuded oocytes were fulfilled under a suitable culture condition. We suggest that oocytes that are already denuded, before or during oocyte collection, have the potential to be matured in vitro if the denuded oocytes are cultured in a suitable culture media.