Spindle imaging in human oocytes: the impact of the meiotic cell cycle

Recent studies using polarized light microscopy revealed a correlation between the presence of a spindle in human metaphase II meiotic oocytes and the fertilization rate following intracytoplasmic sperm injection (ICSI). Using a new spindle imaging system, it was possible to visualize the spindle image and the conventional light microscopic view of the oocyte simultaneously. Using this system, time-lapse studies of the meiotic cycle of human oocytes were performed. The video sequences showed that during the transition from metaphase I to metaphase II, the spindle completely disappears for approximately 40-60 min. These data support the idea that at least in some oocytes, the absence of the spindle is more likely an indicator for physiological progression through an important developmental stage of meiosis rather than a cellular disturbance. In view of the low fertilization rates of oocytes with absence of spindles as reported in the literature, the underlying problem could simply be the incorrect timing of ICSI.