Microtubules are much more rigid than actin molecules and have a larger diameter: 25 nanometers. One end of each microtubule is embedded in the centrosome; the microtubule grows outward from there. Microtubules are relatively unstable and go through a process of continuous growth and decay. Centrioles are small arrays of microtubules that are found in the center of a centrosome. Certain proteins will use microtubules as tracks for laying out organelles in a cell.
Intermediate filaments are the final class of proteins that compose the cytoskeleton. These structures are rope-like and fibrous, with a diameter of approximately 10 nanometers. They are not found in all animal cells, but in those in which they are present they form a network surrounding the nucleus often called the nuclear lamina. Other types of intermediate filaments extend through the cytosol. The filaments help to resist stress and increase cellular stability.
These three types of protein are distinct in their structure and specific function, but all work together to help provide intra-cellular structure. Because they are so diverse, it is very difficult to study the specific functions of the cytoskeletal components.