A human autoantiserum (5051) directed against pericentriolar material (PCM) was used to study the distribution of microtubule-organizing centers (MTOCs) in the oocyte and during the first cell cycle of mouse development. In oocytes, the PCM was found not only at the poles of the barrel-shaped metaphase II spindle but also at many discrete loci around the cytoplasm near the cell cortex. The spindle poles were also composed of several PCM foci. In metaphase-arrested eggs only the PCM foci located near the chromosomes acted as MTOCs. However, after reduction of the critical concentration for tubulin polymerization by taxol, the cytoplasmic PCM foci were also found to be associated with nucleation of microtubules. After fertilization the cortical PCM foci remained in a peripheral position until the end of the S phase, when they appeared to migrate centrally towards the pronuclei. At prometaphase of the first mitotic division, numerous MTOCs were found around the two sets of chromosomes; these MTOCs then aligned to form two bands on either side of the metaphase plate of the first mitosis.

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