Time course of meiotic spindle development in MII oocytes

The aim of this study was to examine changes in meiotic spindle morphology over time to potentially optimize timing for ICSI. Using polarized light microscopy, images of MII oocytes were captured after retrieval of oocytes in stimulated cycles at six time intervals in culture: 36-36.5 h, 36.5-37.0 h, 38-38.5 h, 39-39.5 h, 40-40.5 h and 40.5-41 h post hCG. Captured images were analysed for spindle presence and their retardance. Results showed that spindles were detected in 58% (45/78) of oocytes at 36-36.5 h. This percentage rose to a peak (96% vs. 58%, p < 0.001) at 39-39.5 h and stabilized between 39-40.5 h post trigger then significantly declined at 40.5-41 h post hCG (96% vs. 77%, p < 0.001). Average spindle retardance increased from 36-36.5 h (1.8 ± 0.7 nm) until it peaked at 39-40.5 h (3.8 ± 0.8 nm, p < 0.0001) and then declined significantly after 40.5-41 h (3.2 ± 0.9 nm, p = 0.0001). These results show that the meiotic spindle appearance is time dependent with the majority of oocytes having detectable spindles and highest retardance between 39-40.5 h post hCG under currently used stimulation protocol after which they start to disaggregate. 39-40.5 h post hCG may be the optimal time for ICSI.