Reversing cytoplasmic flow in nucleated, constricted sand dollar eggs

Abstract

AbstractThe purpose of this investigation was to study the nature of the immediate response of the surface to the mitotic apparatus in sand dollar (Echinarachnius parma) eggs. The normal geometrical relationship was changed by snaring fertilized eggs between opposed, overlapping hooks and constricting the eggs to reshape them into two partial spheres joined by a constricted neck about 27 μm in diameter. When both asters of the mitotic apparatus lay on the same side of the constriction, cytoplasm flowed out of the nucleated part across the constriction, beginning about 30 min before anticipated cleavage time. After the asters moved across the constriction, the direction reversed, and reversal usually occurred several times before cleavage (n = 18). The average time between beginning of flow in one direction and beginning of flow in the opposite direction was 7.7 min (n = 23). Cells were bisected and constricted to determine the role of the mitotic apparatus. Bisected cells with two asters behaved like constricted normal cells (n = 15). Cells with no mitotic apparatus developed no cytoplasmic flow (n = 11). In cells with one aster, flow most often occurred within ten min of anticipated cleavage time (n = 14). When whole constricted eggs were treated with Cytochalasin B at the minimum concentration that blocks cleavage in normal eggs (3 μg/ml), neither cytoplasmic flow nor cleavage occurred, although the mitotic apparatus developed normally.Cytoplasmic flow in constricted eggs in cleavage stages requires asters and functional actin‐associated cytoskeleton. The results suggest that the asters cause an increase in tension in nearby surfaces.