Formation of miniasters in the cytoplasm of hexyleneglycol‐treated sea urchin eggs

Miniasters formed in mitotic sea urchin egg after treatment with 5% hexylene-glycol were investigated with the combined techniques of indirect immunofluo-rescence using anti-tubulin and anti-51 kD protein antibodies and electron microscopy. The formation of miniasters was dependent on the mitotic cycle. In the cytoplasm of eggs treated with hexyleneglycol at early prometaphase, a small number of microtubule fragments was observed, whereas in those treated at pro-metaphase, many miniasters and microtubule fragments were seen. When treated at metaphase, we found a great number of miniasters: 250–350 in one egg. In contrast, no miniasters were seen in eggs treated at anaphase, although many long microtubules that spread throughout the cytoplasm were observed. In the eggs treated at telophase, we scarcely noticed microtubule structures in the cytoplasm. In the center of miniasters, granules were found, showing the same size and electron density as those of the microtubule-organizing granules (MTOGs). Furthermore, the 51 kD protein, a component of the centrosome and mitotic spindle, was observed to be localized in the region of miniasters.