 The outer diameter of a microtubule is between 23 and 27 nm while the inner diameter is between 11 and 15 nm.They are formed by the polymerization of a dimer of two globular proteins, alpha and beta tubulin into protofilaments that can then associate laterally to form a hollow tube, the microtubule.The most common form of a microtubule consists of 13 protofilaments in the tubular arrangement.