Soon after cell division, new cells form a cytoplasmic network of microtubules #1, #2, #3 with a marked radial symmetry. The microtubules are made up of dimers of the protein tubulin. The microtubule network formation is initiated in cytocenters comprising a pair of centrioles inside amorphous pericentriolar material. One of the centrioles is obliquely cut (see the image). On the hwole, this cytoplasmic zone is defined as the microtubule-organizing center (MTOC). The growing microtubule (+) ends are directed to the cell periphery. They may branch and form transverse bonds and thus a large network permeating the cell cytoplasm. The network is dynamic and is able to rearrange depending on conditions and needs of the cell.

The microtubule cell network completes two principal tasks. First, it upholds the cell shape and renders the cell stiff. Second, cells use microtubules to guide the transport and distribution of molecules and organelles (e.g., mitochondria) inside them. Synthesized by the endoplasmic reticulum, proteins are then packaged by Golgi apparatus into membrane containers and so move further to their destination via microtubules. There is also reverse or endocytic trafficking whose vectors are directed to the centrosome. The corresponding membrane structures, partially clathrin-coated, are seen in the image. The cistern stacks of Golgi apparatus are found near the nucleus and around the cytocenter; together with the latter, they form a central sorting station to control the bilateral intracellular trafficking. Two motor proteins are used to move the containers and organelles via the microtubule network, and these are kinesin and dynein.