Differential behavior of centrosomes in unequally dividing blastomeres during fourth cleavage of sea urchin embryos.

The fourth cleavage in sea urchin embryos is unequal and represents an initial step in cell differentiation: vegetal blastomeres divide to produce micromeres, which are precursors of the skeletogenic mesenchyme, and macromeres. The mitotic spindles of these unequally dividing cells are peripherally located and lie orthogonal to the vegetal pole plasma membrane; the aster of the micromere pole is closely apposed to the plasmalemma and presents a characteristic flattened profile. In order to investigate the role of centrosomes in the generation of the asymmetric vegetal spindle at fourth cleavage, structural dynamics of centrosomes in both equally and unequally dividing blastomeres were compared using immunofluorescence methods. Quantitation of immunofluorescence and three-dimensional reconstruction techniques demonstrate that micromere centrosomes differ from macromere centrosomes in two respects: (1) micromere spindle poles contain less centrosomal material than macromere poles, and (2) micromere centrosomes undergo a specific filiform elongation during late anaphase and telophase. The behavior of micromere centrosomes suggests that a unique spindle pole event, involving interactions of the microtubular cytoskeleton, centrosome and cell cortex, occurs during the process of unequal cleavage of vegetal blastomeres.