Dynamics of microtubules and positioning of female pronucleus during bovine parthenogenesis

The zygote centrosome, consisting of both paternal and maternal centrosomal components, is the microtubule-organizing center necessary for pronuclear migration and positioning in fertilization. Maternal centrosomal function in microtubule organization and pronuclear positioning, however, remains unclear. In the present study, we sought to elucidate the function of maternal centrosomes during bovine parthenotes in the microtubule organizational processes required to move the pronucleus to the cell center without sperm centrosomal components. Microtubule organization, pronuclear position, and distribution of gamma-tubulin, which is thought to be the major component of maternal centrosomal material, were imaged by immunocytochemistry and conventional epifluorescence microscopy. In bovine parthenotes treated with paclitaxel, a microtubule-stabilizing drug, the cytoplasmic microtubule asters became organized after chemical activation, and the microtubules radiated dynamically toward the female pronucleus. The microtubule patterns correlated well with pronuclear movement to the cell center. Microtubules aggregated at regions of gamma-tubulin concentration, but gamma-tubulin did not localize to a spot until the first interphase of bovine parthenogenesis. These findings indicate that gamma-tubulin is responsible for microtubule organization as the maternal centrosome. In bovine parthenogenesis, the maternal centrosome then organizes cytoplasmic microtubules to move the female pronucleus into the cell center. We propose that the maternal centrosome plays a role as a functional centrosome despite the lack of a sperm contribution, making this structure less competent for microtubule organization in comparison with centrosomes containing sperm centrosomal components.