The coelomocytes of the sea urchin, Strongylocentrotus droebachiensis, may be prevented from clotting with 50 mM ethylene glycol-bis(/3-aminoethyl)-

No abstract

Isolated petaloid coelomocytes from the sea urchin Strongylocentrotus droebachiensis transform to a filopodial morphology in hypotonic media. Electron micrographs of negatively stained Triton-insoluble cytoskeletons show that the petaloid form consists of a loose net of microfilaments while the filopodial form consists of paracrystalline bundles of microfilaments . Actin is the major protein of both forms of the cytoskeleton . Additional polypeptides have molecular weights of 220,000, 64,000, 57,000, and 27,000 daltons . Relative to actin the filopodial cytoskeletons have an average of 2 .5 times as much 57k polypeptide as the petaloid cytoskeletons.Treatment with 0 .25 M NaCl dissociates the filament bundles into individual actin filaments free of the actin-associated polypeptides . Thus, one or more of these actin-associated polypeptides may be responsible for crosslinking the actin filaments into bundles and maintaining the three-dimensional nature of the cytoskeletons .

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The motion of particles in the axopodia of Echinosphaerium nucleofilum is saltatory. In the present study, photokymograph records of 123 motions from six axopodia have been analyzed. Particles followed rectilinear paths of from 1 to 15 am while in continuous motion at an average velocity of 0.66 • 0.32 #m/s. The velocity of the particles was variable in 36% of the cases measured. Some motions were punctuated by pauses either before continuing in the same direction or reversing. Frequently, several particles were moving at the same velocity, but neighboring particles showed no motion or moved in the opposite direction. Two particles occasionally contacted one another and travelled as a unit for varying lengths of time but subsequently moved independently. These motions reflect the underlying mechanism of motive force production. Furthermore, a glass microneedie can be substituted for the microtubular axoneme in the axopodia. In these artificial axopodia, bidirectional particle motions occurred which were similar to those in normal axopodia. Colchicine, at the threshold dose for axonemal dissolution, had no affect on these particle motions. It is concluded that the microtubular axoneme is not responsible for particle motions and also that individual microtubules are unlikely candidates for motive force production in this system. The axopodia of Echinosphaerium nucleofilumare needle-like projections up to 300/~m in length that radiate from the cytosome and are supported longitudinally by two coiled sheets of cross-linked microtubules, the axoneme (5, 9, 12). In Echinosphaerium the prevalence of cytoplasmic microtubules has suggested a possible role for microtubules in intracellular motility. The majority of ultrastructural studies have shown microtubules to be the only linear element present in the cytoplasm. However, Hovasse (4) has reported "skeins of microfibrils," as well as microtubules, to be present in the axopodia, an observation that more recent works have not confirmed. Thus, since alternatives are lacking, cytoplasmic motility has been attributed to the microtubules.The axopodia constitute a convenient system for studying particle motions because their geometry confines the motions to the long axis of the axopodia. The one-dimensional motions are readily analyzed with a photokymograph.