Aberrant spindle structures responsible for recurrent human metaphase I oocyte arrest with attempts to induce meiosis artificially

BACKGROUND In some couples, not all retrieved oocytes mature, even after prolonged in vitro culture. The underlying mechanisms are not known, although ionophore treatment may alleviate metaphase I (MI) arrest in some mouse strains. We attempted to induce first polar body (PB) extrusion and fertilization using assisted oocyte activation (AOA) after ICSI in maturation-resistant human MI oocytes. METHODS Four ICSI patients are described in this retrospective study. A pilot study tested the calcium ionophore ionomycin (10 µM) on donated MI oocytes from patients with a normal number of metaphase II (MII) oocytes. Subsequently, ionomycin was used to induce first PB extrusion in two patients showing maturation-resistant MI oocytes. AOA, by calcium injection and ionomycin exposure, was applied when mature oocytes were available. Oocytes were analysed by polarized microscopy and immunostaining. RESULTS Ionomycin induced the first PB extrusion in MI oocytes from patients with a normal number of retrieved MII oocytes, while extended in vitro culture failed to achieve the MII stage. Similarly, ionomycin induced first PB extrusion in one of two patients with recurrent maturation-resistant MI oocytes. Use of ICSI combined with AOA on MII oocytes matured in vitro or in vivo resulted in failed or abnormal fertilization with no further embryo cleavage potential. Highly abnormal spindle and chromosome configurations were observed in MI maturation-resistant oocytes, in contrast to control MI oocytes. CONCLUSIONS Ionophore induced first PB extrusion in MI oocytes from patients without maturation arrest but to a lower extent in maturation-resistant MI oocytes. Immunofluorescence staining and confocal analysis revealed, for the first time, highly abnormal spindle/chromosomal structures that may be responsible for this maturation arrest.