Nuclear Migration and Spindle Formation in the Fourth Cleavage of Sea Urchin Eggs under the Influence of Inhibitors

Abstract

Mitosis of sea urchin eggs was inhibited when exposed to 3μg/ml aphidicolin from the 2-celI stage onwards. Nevertheless the nuclei migrated to the vegetal pole at the time of the fourth unequal division in control eggs. Two or four equal or unequal asters developed. Asters in proximity to the vegetal pole were always considerably smaller than those close to the center of the two blastomeres. In contrast to colchicine, cytokinesis but not migration of the nuclei in the vegetal half was prevented by treatments with 5 μM cytochalasin B or D. Various mitotic figures were formed in the vegetal blastomeres of eggs treated with 0.4 mM colchicine or 3 μM griseofulvin after the third cleavage. In some eggs a centrally localized monaster with chromosomes in spherelike arrangement was formed in others a monopolar mitotic figure pushed the chromosomes in bowl-like arrangements to the most vegetal cortex. In anaphase one set of chromatids migrated to the monopole leaving the scattered sister-chromatids behind. The mechanism of migration of the nuclei and of chromosome arrangement in the metaphase plate is discussed. © 1992, Japan Society for Cell Biology. All rights reserved.