Microinjected carboxylated beads move predominantly poleward in sea urchin eggs

Observations on living mitotic cells have suggested that material in the spindle moves poleward during mitosis. In order to investigate this movement, sea urchin eggs have been microinjected with 0.25-micron diameter carboxylated fluorescent beads. When fluorescent beads were injected into unfertilized Lytechinus variegatus eggs, no motility was detected. When injected into mitotic cells, beads moved to the spindle poles. Individual beads moved rapidly, in a saltatory fashion, and followed generally linear paths. Beads appeared to move along astral fibers, were generally excluded from the spindle proper, and accumulated at the spindle poles. Some dispersion of the beads away from the pole was observed as cells completed mitosis, but the majority of beads retained a polar location. After depolymerization of spindle microtubules with nocodazole, some dispersion of beads into the cytoplasm was also observed. Beads moved along taxol-induced astral microtubules and accumulated at astral centers. These observations reveal that negatively charged beads accumulate rapidly at mitotic centers, moving toward the minus end of the microtubules. Neither the bidirectional motility of similar beads in interphase cells nor the plus-end-directed bead motility seen in axons was observed in these mitotic cells.