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Microtubule

(Science: cell biology) cytoplasmic tubule, 25nm outside diameter with a 5nm thick wall. Made of tubulin heterodimers packed in a three start helix (or of 13 protofilaments looked at another way) and associated with various other proteins (mAPs, dynein, kinesin). Microtubules of the ciliary axoneme are more permanent than cytoplasmic and spindle microtubules.