Microtubule organization in porcine oocytes during fertilization and parthenogenesis.

Microtubule configurations in porcine oocytes after sperm penetration or after artificial activation by electrical stimulation were imaged by immunocytochemistry and laser scanning confocal microscopy. Soon after sperm penetration, an aster was seen adjacent to the incorporated sperm head. Polyspermic penetrations led to the presence of multiple sperm asters in association with each sperm. The sperm aster enlarged and, at the time of pronuclear apposition, filled the cytoplasm. After male and female gamete union, the microtubule matrix was reduced. At the mitotic metaphase stage, microtubules were detected in the spindle, which was anastral and fusiform. At anaphase, asters assembled at each spindle pole, and at telophase, large asters filled the cytoplasm. Artificial activation by electrical stimulation induced in the cytoplasm a dense network of microtubules, which seem to be involved in proper positioning of the female pronucleus. At mitotic metaphase, microtubules were concentrated around the chromatin. The results of experiments using taxol, a microtubule stabilizing agent, suggest that maternal centrosomal material is present in the mature porcine oocyte as dispersed undetectable material that can form a microtubule network after parthenogenetic activation. However, at fertilization, the paternal centrosome collects centrosomal material to form a sperm aster. These results suggest that the functional centrosome that forms during fertilization is a result of the blending of paternal and maternal centrosomal components.