While microtubule (MT) arrays in cells are often focused at the centrosome, a variety of cell types contain a substantial number of noncentrosomal MTs. Epithelial cells, neurons, and muscle cells all contain arrays of noncentrosomal MTs that are critical for these cells' specialized functions. There are several routes by which noncentrosomal MTs can arise, including release from the centrosome, cytoplasmic assembly, breakage or severing, and stabilization from noncentrosomal sites. Once formed, MTs that are not tethered to the centrosome must be organized, which can be accomplished by means of self-organization or by capture and nucleation of MTs where they are needed. The presence of free MTs requires stabilization of minus ends, either by MT-associated proteins or by an end-capping complex. Although some of the basic elements of free MT formation and organization are beginning to be understood, a great deal of work is still necessary before we have a complete picture of how noncentrosomal MT arrays are assembled in specific cell types.
