Development of the ear

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CHAPTER 38 Development of the ear

INNER EAR

The production of a precisely positioned and functionally well tuned inner ear depends on genetic patterning and a cascade of transcription signals expressed by numerous tissues, including the developing inner ear and its surrounding periotic mesenchyme, the adjacent hindbrain, neural crest and notochord (e.g. Liu et al 2003).

The first signs of inner ear development are visible shortly after those associated with the developing eyes. Two patches of ectodermal thickening, the otic placodes, appear lateral to the hindbrain at stage 9. Each otic placode invaginates as an otic pit, adjacent to rhombomeres 5 and 6 of the hindbrain and dorsal to the second pharyngeal cleft. The pit becomes pinched off from the surface ectoderm to form a simple epithelial sac, the otocyst (auditory or otic vesicle), during stage 12 (Fig. 38.1).

The first morphological differentiation of the otic vesicle is visible during stage 14 (approximately 33 days) and presages the three major subdivisions of the inner ear, i.e. the endolymphatic sac and duct, the dorsal utricular portion that later gives rise to the semicircular canals, and the ventral saccular portion that later gives rise to the cochlea. At this time the otocyst is piriform in shape. A tubular diverticulum develops from its medial rim and differentiates into the endolymphatic duct and sac. The sac communicates via the endolymphatic duct with the remainder of the otocyst which is placed laterally and can be regarded as the utriculosaccular chamber. At the same time, two plate-like diverticulae, one vertical and one horizontal, emerge from the dorsal part of the utriculosaccular chamber. The opposing epithelia in the central region of each outgrowth coalesce to form a fusion plate, and the central part of this plate is eventually resorbed, producing a tubular canal. The vertical plate gives rise to the anterior and posterior semicircular canals that share a common crural attachment to the utriculosaccular chamber, and the horizontal plate gives rise to the horizontal or lateral semicircular canal. A medially directed evagination, the cochlear anlagen, is evident in the ventral part of the utriculosaccular chamber in a 7–9 mm (approximately 35 days) embryo. The proximal region of the cochlear duct continues to increase in length and its distal region becomes progressively more coiled. When the duct has achieved its final length and spiral configuration, its proximal part becomes constricted, forming the ductus reuniens.

The central part of the utriculosaccular chamber now represents the membranous vestibule, which becomes divided into a small ventral saccule and a larger dorsal utricle. This is achieved mainly by horizontal infolding that extends from the lateral wall of the chamber towards the opening of the endolymphatic duct until only a narrow utriculosaccular duct remains between the saccule and utricle. The duct becomes acutely bent on itself: its apex is continuous with the ductus endolymphaticus. During this period the membranous labyrinth rotates so that its long axis, which was originally vertical, becomes more or less horizontal.

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