What is the arrangement and number of protofilaments in a microtubule?

A standard microtubule has 13 protofilaments arranged in a hollow cylindrical lattice with a diameter of about 25 nanometers. Each protofilament is a linear chain of alternating α- and β-tubulin subunits. 

These protofilaments align parallel to the long axis of the microtubule and interact laterally in a staggered manner, giving the tube a helical appearance. The slight offset between protofilaments creates a seamless surface and structural integrity. The 13-protofilament configuration is the most common and energetically stable, though certain specialized structures like cilia or flagella can have modified arrangements. 

The outer wall of the microtubule serves as a track for motor proteins, while the inner lumen remains empty. This specific arrangement allows flexibility, strength, and dynamic growth—essential for mitotic spindle formation, intracellular transport, and maintenance of cell architecture.