The ear consists of three parts that have different origins, but that function as one unit. The internal ear originates from the otic vesicle, which in the fourth week of development detaches from surface ectoderm. This vesicle divides into a ventral component, which gives rise to the saccule and cochlear duct, and a dorsal component, which gives rise to the utricle, semicircular canals, and endolymphatic duct (Figs. 16.3, 16.6, and 16.8). The epithelial structures thus formed are known collectively as the membranous labyrinth. Except for the cochlear duct, which forms the organ of Corti, all structures derived from the membranous labyrinth are involved with equilibrium.
The middle ear, consisting of the tympanic cavity and auditory tube, is lined with epithelium of endodermal origin and is derived from the first pharyngeal pouch. The auditory tube extends between the tympanic cavity and nasopharynx. The ossicles, which transfer sound from the tympanic membrane to the oval window, are derived from the first (malleus and incus) and second (stapes) pharyngeal arches (Fig. 16.9).
The external auditory meatus develops from the first pharyngeal cleft and is separated from the tympanic cavity by the tympanic membrane (eardrum). The eardrum consists of (a) an ectodermal epithelial lining, (b) an intermediate layer of mesenchyme, and (c) an endodermal lining from the first pharyngeal pouch.
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