Changes in centrosomal domains during meiotic maturation in the human oocyte

The distribution of microtubule organizing centres (MTOC) in the human oocyte was examined using the microtubule-active drug, taxol, to promote polymerization. Oocytes were obtained from gonadotrophin-stimulated in-vitro fertilization (IVF) patients and examined during various phases of meiotic maturation using confocal fluorescence microscopy. During the prophase of meiosis I, taxol failed to stimulate microtubule nucleation in any region of the cells. Only a few microtubules were visible in the oocyte cortex. As the transition from prophase to metaphase began, during germinal vesicle breakdown, taxol stimulated the appearance of a small number of isolated aster-like arrays of microtubules in the cortex, predominantly in regions adjacent to the nucleus. Oocytes treated with taxol when they had reached the first meiotic metaphase exhibited a large number of aster-like microtubule arrays. These structures were predominantly located in the cortical region of the oocyte, but smaller arrays were also visible in the endoplasmic areas. There did not appear to be any increased density of asters in any particular cortical or endoplasmic region. Oocytes treated with taxol during the second metaphase of meiosis exhibited a similar response to the drug as seen during metaphase I, Again, the microtubule asters were most prevalent in the cortex, with smaller, less dense arrays in the endoplasm. The metaphase spindle was also affected by taxol as revealed by increased density and hyperelongation of microtubules at the poles of the spindle as compared to untreated oocytes. The metaphase plate of chromosomes was highly disrupted by taxol treatment, most likely due to the forces generated by microtubule elongation. We conclude that the human oocyte develops MTOC as meiotic maturation proceeds beyond the prophase I arrest. The first MTOCs are perinuclear, but the number and distribution increases widely as the oocytes enter metaphase. We hypothesize that the centrosome of the human recruits several MTOC domains for the assembly of the meiotic spindles in both meiotic divisions. It is speculated that one or more of the non-spindle-associated MTOCs may combine with sperm centrosomal material during fertilization to create the complete centrosome needed for embryonic mitosis. The widespread distribution of MTOC foci throughout the cortex may ensure this recombination regardless of the point of sperm incorporation into the oocyte.