Final answer:
Microtubules are composed of multiple protofilaments, with 13 protofilaments usually found in each microtubule.
Step-by-step explanation:
Microtubules are the thickest cytoskeletal structures made of filaments. Each microtubule is composed of multiple protofilaments, which are formed by rows of tubulin subunits.
Normally, there are 13 protofilaments per microtubule.
For example, microtubules in the cilia and flagella of cells are made up of multiple protofilaments that work together to allow movement.