Development of the Ear - Overview

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The major components of the ear are located within the petrous part of the temporal bone. The external ear, which gathers sound waves and funnels them into the auditory canal to the tympanic membrane. The middle ear, which conducts the sound waves internally and the inner ear, which consists of the cochlea, the sensory organ that converts sounds into neural signals that are transmitted to the CNS. The vestibular organ, the organ of balance or equilibrium is also part of the inner ear and develops in concert with the cochlea.

Internal Ear

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    The otic placode appears around day 21 of development and by day 22 it is a thickening of the surface ectoderm in the region of the developing neural tube (hindbrain}.

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    The otic placode invaginates into the surface ectoderm in the region of the hindbrain (rhombencephalon) to form the otic pit.

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    Around day 27, the otic pit begins to pinch off from the surface ectoderm.

    By day 30,     otic pit is separated from the surface ectoderm to become the otic vesicle (otocyst) adjacent to the hindbrain.

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    Some cells of the otic vesicle differentiate into sensory neurons that will grow processes into the hindbrain to make synaptic connections with auditory and vestibular nuclei.

    For the most part, however, the     otic vesicle becomes the convoluted, membranous labyrinth that contains the sensory end organs for balance and hearing.

    The first pharyngeal cleft and pouch form the tympanic membrane or ear drum, and linings of the external ear canal and pharyngotympanic tube and middle ear cavity.

    The     head mesenchyme will give rise to the ear ossicles.
    All of these elements will miraculously combine to produce the functional ear.

The otic vesicle expands regionally to form a utricle and saccule and endolymphatic sac.

In the 5th week, a tubular outgrowth of the saccule begins the development of the cochlear duct. In the next weeks, the cochlear duct forms two and a half spiral turns, similar to a snail shell. Within the cochlear duct a specialized sensory epithelium differentiates, the organ of Corti.

While the cochlear duct is developing from the saccule, a set of three semicircular canals form as out growths of the utricle.

The semicircular canals (superior, lateral and posterior) along with the saccule and utricle constitute the portion of the inner ear devoted to equilibrium or balance. These are connected to the vestibular division of cranial nerve VIII.

The cochlear duct with the organ of Corti is the part of the inner ear devoted to hearing and is connected to the auditory division of cranial nerve VIII.

All these structures, derived from the otic vesicle, collectively make up the membranous labyrinth.

Head mesenchyme surrounding the membranous labyrinth undergoes endochondral ossification to form the bony labyrinth.

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As the cochlear duct forms 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 mesenchyme surrounding the duct differentiates into cartilage. Some of the cartilage breaks down forming two perilymphatic spaces, the scala tympani and the scala vestibuli.

The original cochlear duct forms the scala media.

A bar of cartilage persists between the scala tympani and the scala vestibuli. This forms the modiolus, the central axis of the cochlea.

Endochondral ossification of the remaining cartilage forms the bony labyrinth

Middle Ear

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The middle ear is the sound conducting part of the ear. Sound waves cause vibrations of the tympanic membrane. The tympanic membrane is formed from ectoderm of the first pharyngeal groove, endoderm of the first pharyngeal pouch and an intervening layer of mesoderm.

These vibrations are transmitted to the inner ear by three ear ossicles, malleus, incus and stapes, which form from neural crest derived cartilages of the first and second branchial arches. The middle ear cavity and pharyngotympanic tube are lined by epithelium derived from endoderm of the first pharyngeal pouch.

External Ear

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The external ear forms from six primordia that arise from the 1st and 2nd pharyngeal arches. These six “hillocks” flank the anterior and posterior sides of the 1st pharyngeal groove, which will become the external auditory meatus or auditory canal.
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 anomalies in organs that also include contributions by neural crest, such as the face, skull and heart and GI tract.

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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Clinical Correlates

Congenital Hearing Loss

Sensorineural hearing loss is associated with failure of the auditory nerve or hair cells to develop properly. Conductive hearing loss is related to failure of the eardrum, ear canal or ear ossicles to form properly.

A variety of genetic and environmental causes have been associated with congenital loss of hearing.
Genetic causes include, but are not limited to, Treacher Collins Syndrome, malformation of the ear canal and or ear ossicles are known causes of conductive hearing loss. Prenatal infections with rubella, cytomegalovirus or herpes simplex are environmental factors known to cause sensorineural hearing loss.

In Down syndrome, hearing loss can be related to either sensorineural or conductive causes.