Final answer:
Microtubules in nerve cell axons form tight bundles due to the polymerization of tubulin, which assembles into robust structures essential for maintaining axonal integrity.
Step-by-step explanation:
The tight bundling of microtubules in nerve cell axons is caused by the polymerization of tubulin. Microtubules are composed of α- and β-tubulin protein monomers that assemble into long, stiff structures which can resist compression and serve as tracks for motor proteins. The process of tubulin polymerization facilitates the formation of these microtubular structures, enabling them to maintain the shape and support in the long axons of nerve cells. Tubulin is the primary component of microtubules, which are critically involved in various aspects of cellular motion, including the transport of materials within cells, and are also the main structural element of various cellular components such as centrioles, flagella, and cilia.