Development of the Ear

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There are three components that make up the ear. The external ear, which gathers sound waves. The middle ear, which conducts the sound waves internally and the inner ear, which consists of the sensory organ that converts sounds into electrical neural signals that are transmitted to the CNS. The organ of balance or equilibrium also develops in consort with the organ of hearing. Aspects of the development of the middle ear have been covered in the development of the branchial arches.

Internal Ear

The otic placode appears around day 21 of development. By day 22 it is a thickening of the surface ectoderm in the region of the developing hindbrain.

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The otic placode begins to invaginate into the surface ectoderm in the region of the hindbrain (rhombencephalon) to form the otic pit. Eventually, by about day 30, otic pit is separated from the surface ectoderm and becomes the otic vesicle adjacent to the hindbrain.

  • Endolymphatic Duct

    Through further development, the otic vesicle becomes expanded in some parts to form a utricle and saccule and endolymphatic duct. In the 5th week, a tubular outgrowth of the saccule begins the development of the cochlear duct. Successive stages of development of the cochlear duct are observed in figures (6 weeks), (7 weeks) and (8 weeks). The duct forms two and a half spiral turns, similar to a snail shell. Within it, the cells of the organ of Corti differentiate. These components collectively make up the membranous labyrinth. The mesenchyme surrounding the membranous labyrinth undergoes endochondral ossification to form the bony labyrinth.

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  • Cochlear Duct
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    Initially, the cochlear duct is surrounded by mesodermal mesenchyme, which eventually forms cartilage. As the cochlear duct forms its spirals to develop into the cochlea, the epithelial cells begin to differentiate into sensory cells of the organ of Corti. Around the 10th week, the cartilage surrounding the duct begins to break down to form two perilymphatic spaces, the scala tympani and the scala vestibuli. The original cochlear duct forms the scala media. These fluid filled spaces form the membranous labyrinth. A bar of cartilage persists between the scala tympani and the scala vestibuli. This forms the modiollus, the central axis of the cochlea. Cells that bud off from the epithelium of the cochlear duct differentiate into bipolar ganglion cells to form the spiral ganglion of the auditory portion of cranial nerve VIII.

  • Semicircular Canals

    While the cochlear duct is developing from the saccule, a set of three semicircular canals begins to form as out growths of the utricle during the 7th week of development. The semicircular canals (superior, lateral and posterior) along with the saccule and utricle constitute the portion of the inner ear devoted to equilibrium. Similar to the case of the cochlear duct, the mesenchyme surrounding the semicircular canals ossifies to contribute to the bony labyrinth.

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Middle Ear

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The middle ear is the sound conducting part of the ear. Laterally, the tympanic membrane or eardrum, limits the middle ear cavity. The tympanic membrane is formed from ectoderm of the first branchial groove, endoderm of the first pharyngeal pouch and an intervening layer of mesoderm.
Sound waves strike the tympanic membrane and cause it to vibrate. These vibrations are transmitted to the inner ear by three small bones or ear ossicles. These are from lateral to medial, the malleus, incus and stapes. Developmentally, these bones are from the cartilages of the first and second branchial arches. The middle ear cavity is connected to the nasopharynx by the pharyngotympanic tube, the lining of which is derived from the endoderm of the first pharyngeal pouch. Endoderm from the first pharyngeal pouch also forms the lining of the middle ear cavity.

External Ear

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The external ear forms from six primordia that arise from the 1st and 2nd branchial arches. These six “hillocks” flank the anterior and posterior sides of the 1st branchial groove, which will become the external auditory meatus.
These primordia are covered by ectoderm, but also consist of tissue of neural crest origin. Formation and development of the external ear is complex and for this reason, developmental anomalies of the ear are quite common.
Because of their origins from neural crest tissue, malformations of the external ear may signal other anomalies in organs that also form from neural crest, such as the face, skull and heart.

The adult external ear possesses several identifiable parts, here correlated with their origins from the original six primordial that gave rise to the external ear.

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