What structure in cells is primarily composed of microtubules?

The spindle is primarily composed of microtubules, which are dynamic structures made from tubulin proteins. During cell division, specifically in the processes of mitosis and meiosis, the spindle apparatus forms to facilitate the separation of chromosomes. Microtubules extend from centrosomes at opposite poles of the cell and attach to the kinetochores of chromosomes, pulling them apart to ensure that each daughter cell receives an equal and accurate complement of genetic material. This role of microtubules is essential for maintaining genomic stability during cell division, showcasing their critical function in cellular organization and division processes.