The hair system provides sensory information to the vestibular and auditory systems. The hair system also protects against abrasion and exposure to UV radiation. Hair cells are sensory cells capped with filamentous structures, the stereocilia, that line the cochlea of vertebrates. The set of hair cells and the membranes attached to them constitute the organ of Corti. Dermal papilla cells, germinal matrix cells, inner and outer root sheath cells and follicular keratinocytes are essential components for normal hair development and growth. Abnormal cell regulation within the hair system contributes to the development of disorders such as alopecia.