Microtubules are made of a protein called "tubulin". The tubulin is "polymerized", which is when molecules form the same shapes over and over again that are linked together in groups, and these groups are linked together. They can form long chains or other shapes; in this case the polymerized tubulin forms microtubules. The microtubules are rigid tubes like microscopic straws which are hollow inside. Microtubules help keep the shape of the neuron, and are inolved in passing signals through the neuron.