During the differentiation described above, the epithelium of the internal ear vesicle thickens where it is in contact with the ingrowing nerve fibres: in the saccule (macula sacculi), the utricule (macula utriculi), the enlarged ampullae of the semicircular ducts (maculae and cristae ampullares), and along one side of the cochlear duct (spiral organ, or organ of Corti). The cells of these areas differentiate into hair cells and supporting cells. Each of these areas is supplied by sensory fibres of the vestibular and cochlear nerve, respectively.