Reversing cytoplasmic flow in nucleated, constricted sand dollar eggs

Abstract

The 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 Fm 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 pg/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 actinassociated cytoskeleton. The results suggest that the asters cause an increase in tension in nearby surfaces.The idea that the role of the mitotic apparatus in cytokinesis of animal cells is confined to preliminary modification of regional surface properties was postulated in the last century by Biitschli (1876) and then by Ziegler in 1903. Later, demonstrations of the cell's ability to complete division despite removal of the mitotic apparatus (Yatsu, '12; Hiramoto, '56) led to general acceptance of the idea. The nature of the immediate effect of mitotic apparatus action on the surface can not be determined by simple observation, and two alternative mechanisms have been suggested. Lillie ('031, Wolpert ('601, Schroeder ('81), and White and Borisy ('83) proposed that the mitotic apparatus causes a decrease in tension or relaxation of the affected surface, while Biitschli (18761, Ziegler ('03), and many others have proposed that the mitotic apparatus causes an increase in tension or contraction in tfhe affected surface. Either response could lead to the organization of the filamentous contractile ring (Arnold, '68; Schroeder, '681, which, it is generally agreed, bisects the cell by constriction.Measurements of the changes in cytoplasmic stiffness that appear to accompany the cell cycle (Hiramoto, '63; Yoneda and Dan, '72) have not provided a basis for eliminating either of the hypothetical mechanisms. The understanding of the relation between the stiffness changes and cytokinesis is complic...

How an actin network might cause fountain streaming and nuclear migration in the syncytial Drosophila embryo [published erratum appears in J Cell Biol 1995 Sep;130(5):1231-4]

“…The actin cytoskeleton of many embryos, including C. elegans (Strome, 1986) and Boltenia, an ascid-ian (unpublished observations), resembles what we have described in Drosophila: a dense granular cortical layer and a less dense fibrous network extending into the cytoplasm. Cytoplasmic streaming has also been documented in these and other embryos (Hird and White, 1993;Rappaport and Rappaport, 1988; our unpublished observations). Taylor and colleagues have demonstrated the application of solationcontraction coupling not only to amoeba but also to motility of cultured mammalian cells, and have shown that actin polymerizes into a network near the periphery of the interphase cell and disassembles in the vicinity of the nucleus (Kolega et al, 1991;Giuliano and Taylor, 1994).…”

Analysis of Cortical Flow Models In Vivo

“…Rappaport and Rappaport (1988) found that when microtubule asters are trapped within a starfish embryo constricted into a dumbbell shape, flow moves away from the half of the embryo containing the asters. They concluded that the half containing the asters was more contractile and squeezed cytoplasm into the other half (Rappaport and Rappaport, 1988;Rappaport, 1996). Hird and White (1993), on the other hand, found that, in Caenorhabditis elegans embryos treated with nocodazole to create a diminutive aster near the cortex, cortical flow of F-actin moved away from the aster.…”

“…Two studies of flow patterns after microtubule manipulation reached opposite conclusions. Rappaport and Rappaport (1988) found that when microtubule asters are trapped within a starfish embryo constricted into a dumbbell shape, flow moves away from the half of the embryo containing the asters. They concluded that the half containing the asters was more contractile and squeezed cytoplasm into the other half (Rappaport and Rappaport, 1988;Rappaport, 1996).…”

Midzone microtubule bundles are continuously required for cytokinesis in cultured epithelial cells [published erratum appears in J Cell Biol 1996 Dec;135(6 Pt 1):1679]

“…Such convincing studies from Echinoderm eggs have led to a general view that the spindle poles represent the key cleavage plane determinant in all animal cells (for reviews see Mabuchi, 1986;Rappaport, 1991). Furthermore, as the spindle poles are distant from the cortex, the anaphase astral microtubulesrare proposed to transmit the cleavage signal to the cortex (Schroeder, 1981;White and Borisy, 1983;Rappaport and Rappaport, 1988;Harris and Gewalt, 1989;Devore et al, 1989;Rappaport, 1991).…”