Basal bodies are formed either by de novo formation without contact with the pre-existing centrioles (the acentriolar pathway) or by duplication of existing centrioles (the centriolar pathway). About 95% of the centrioles are generated through the acentriolar pathway. Both pathways give rise to multiple immediate precursors of centrioles, known as procentrioles, which mature as they migrate to the appropriate site near the apical cell membrane, where they become basal bodies (Fig. 2.53). The basal body acts as the organizing center for a cilium. Microtubules grow upward from the basal body, pushing the cell membrane outward, and elongate to form the mature cilium. The process of centriole duplication is described later on page 70.
 Mitotic spindle formation. During mitosis, the position location of the MTOC in relation to the nucleus and the Golgi apparatus. In some species, the MTOC is tethered to the nuclear envelope by a contractile protein, the nucleus–basal body connector (NBBC). The MTOC contains the centrioles and an amorphous protein matrix with an abundance of -tubulin rings. Each -tubulin ring serves as the nucleation site for the growth of a single microtubule. Note that the minus (–) end of the microtubule remains attached to the MTOC, and the plus ( ) end represents the growing end directed toward the plasma membrane.
of centrioles determines the location of mitotic spindle poles. Centrioles are also necessary for the formation of a fully functional MTOC, which nucleates mitotic spindle– associated microtubules. For instance, astral microtubules are formed around each individual centriole in a starlike fashion. They are crucial in  establishing the axis of the developing mitotic spindle. In some animal cells, the mitotic spindle itself (mainly kinetochore microtubules) is  FOLDER 2.2 Clinical Correlation: Abnormalities in Microtubules and Filaments
Abnormalities related to the organization and structure of microtubules, actin, and intermediate flaments underlie a variety of pathologic disorders. These abnormalities lead to defects in the cytoskeleton and can produce a variety of defects related to intracellular vesicular transport, intracellular accumulations of pathologic proteins, and impairment of cell mobility.
Defects in the organization of microtubules and microtubule-associated proteins can immobilize the cilia of respiratory epithelium, interfering with the ability of the respiratory system to clear accumulated secretions. This condition, known as Kartagener’s syndrome (see page 120), also causes dysfunction of microtubules, which affects sperm motility and leads to male sterility. It may also cause infertility in women because of impaired ciliary transport of the ovum through the oviduct.