Microtubules are hollow cylinders of tubulin protein in the cytoplasm of all eukaryotic cells. As part of a cell's cytoskeleton, they shape and support it. Microtubules are also involved in the transport of materials inside of the cell. They serve as paths along which organelles with motor-equipped molecules can move. Microtubules participate in the separation of chromosomes during cell division (mitosis) and can act alone or form such complex structures as flagella and cilia.