Centriole as microtubule-organizing centers for marginal bands of molluscan erythrocytes

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Abstract
The erythrocytes of blood clams (Arcidae: Anadara ovalis and Noetia ponderosa) are flattened, elliptical and nucleated. They contain elliptical marginal bands (MB) of microtubules, each physically associated with a pair of centrioles. The MB were cold labile in living cells, disappearing within 1-2 h at 0.degree. C. After the cells had been rewarmed for 1-2 h, continuous MB with associated centrioles were once again present. Time-course studies utilizing phase contrast, antitubulin immunofluoresence and EM of cytoskeletons prepared during rewarming revealed structural evidence of centriole participation in MB reassembly. At the earliest stage of reassembly, a continuous MB was not present. Instead, relatively short and straight microtubules focused on a pointed centriolar pole, and none were present elsewhere in the cytoskeleton. Thin continuous MB then formed, still pointed in the centriolar region. Subsequently, the MB regained ellipticity, with their thickness gradually increasing but not reaching that of controls even after several hours of rewarming. At these later time points, microtubules still radiated from the centrioles and joined the MB some distance away. In the presence of 0.1 mM colchicine, MB reassembly was arrested at the pointed stage. EM observations indicate that pericentriolar material is involved in microtubule nucleation in this system, rather than the centriolar triplets directly. The results suggest a model in which the centrioles and associated material nucleate assembly and growth of microtubules in diverging directions around the cell periphery. Microtubules of opposite polarity would then pass each other at the end of the cell distal to the centrioles, with continued elongation eventually closing the MB ellipse behind the centriole pair.