development of the ear

7/30/2019 Development of the Ear

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Development of the Ear

The development of the structures necessary for transmission of sound information from the

environment to the auditory cortex is a complex and interwoven process. An understanding of

the major developmental steps and their interrelations is desirable because therein lies the key

to understanding many conditions encountered by otolaryngologists. Awareness of the

developmental process alerts the surgeon to anatomic associations and explains importantdepartures from normal.

Abnormal development is important for its clinical effects, but it also has a role in unraveling

the complexities of normal development. The critical period of ear development begins in the

third week after fertilization, the inner ear appearing first. The inner, middle, and outer

portions of the ear have different embryologic origins, and development can be arrested at

any stage. The result is a range of abnormalities from mild to severe. In view of the different

origins, a disorder in one part does not necessarily signify a disorder in another, but proximity

in terms of time of development, originating tissue, anatomic characteristics, and function

does mean that multiple malformations are possible. The disorders can be caused by an

inborn genetic error, either inherited or spontaneous, or by a teratogenic influence during

organogenesis. The tissues of the head and neck are derived from all three layers of theembryoectoderm, mesoderm, and endoderm. The neural crest cells play a special role in

the head and neck, where they constitute most of the skeletal and connective tissue. These

cells arise from the ectodermal layer at the junction where the neural tube begins to fold. All

divisions of the ear contain some neural crest tissue. The mesodermal proportion in the head

and neck is less than that in the rest of the body.

The story of ear development goes back to the time life itself was in its infancy. Fish seem to

be the first hearing organisms, with development of a hearing organ from an internal balance

organ. Even at this early evolutionary stage, the hair-cell design now so widespread was in

use. Both amphibians and reptiles inherited the balance labyrinth of fish but went on to

develop auditory labyrinths of their own, having branched from the line of fish before

acquisition of a hearing organ. The need to hear in air resulted in development of a

conductive apparatus to correct the impedance mismatch of sound arriving in air but having

to be transmitted into the liquid of the labyrinth. Mammalian design continued from the basic

reptilian design with, in particular, the addition of rows of hair cells, an independent cochlear

nerve, changes in the middle-ear conduction system, and protective external auditory canals.

Throughout this work, we separate development of the ear into its component parts as an aid

to understanding. It is important, however, to remember that these changes occur in a

simultaneous manner.

Auricular Development

In keeping with its recent evolutionary appearance, the auricle of the external ear begins itsdevelopment later than do other components of the ear. From the fifth week of gestation,

three hillocks arise on the first branchial (mandibular) arch (hillocks 1, 2, and 3), and three

arise on the second branchial (hyoid) arch (hillocks 4, 5, and 6) on either side of the first

branchial cleft

Hillocks 1 and 6 are the first to be identifiable separately, but by the sixth week, all are

distinct. The lobule also can be identified on the second arch. By the eighth week, the auricle

has an identifiable structure, and the contributions of the hillocks to the adult form can be

recognized: hillock 1, tragus; hillock 2, crus helicis; hillock 3, ascending helix; hillock 4,

horizontal helix, upper portion of scapha, and antihelix; hillock 5, descending helix, middle

portion of the scapha, and antihelix; and hillock 6, antitragus and inferior aspect of the helix.

Although this is the majority view, uncertainty exists about the origin of the crus helicis andascending helix; some believe these structures can arise from the second arch. By


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approximately 18 weeks gestation, the auricle has achieved essentially adult form, although it

continues to grow in childhood with changes continuing into late adult life.

Development of External Auditory Canal

The external auditory canal begins to form in the fourth week of gestation. The first branchialcleft, between the first and second branchial arches, widens, and the ectoderm proliferates to

form a pit, which comes into apposition with the endoderm of the first pharyngeal pouch.

This pit is the forerunner of the cartilaginous external auditory canal. This arrangement is

temporary, because mesenchymal growth separates the cleft and the pouch. The deep portion

of the external auditory canal is apparent from the eighth week of gestation as a strand of

epithelial cells running down to the disk-shaped precursor of the tympanic membrane . At

approximately 28 weeks' gestation, this epithelial core has canalized from the medial to the

lateral aspect to allow communication with the tympanic membrane. The epithelial core is the

precursor of the bony external auditory canal.

External Ear

The auricle (L. auris) is composed of elastic cartilage covered by thin skin. The auricle has

several depressions and elevations. The concha is the deepest depression, and the elevated

margin of the auricle is the helix. The non-cartilaginous lobule (earlobe) consists of fibrous

tissue, fat, and blood vessels. It is easily pierced for taking small blood samples and inserting

earrings. The tragus is a tongue-like projection overlapping the opening of the external

acoustic meatus. The arterial supply to the auricle is derived mainly from the posterior

auricular and superficial temporal arteries. The main nerves to the skin of the auricle are the

great auricular and auriculotemporal nerves, with minor contributions from the facial (CN

VII) and vagus (CN X) nerves.Lymphatic drainage from the lateral surface of the superior half of the auricle is to the

superficial parotid lymph nodes. Lymph from the cranial surface of the superior half of the

auricle drains to the mastoid and deep cervical lymph nodes. Lymph from the remainder of

the auricle, including the lobule, drains to the superficial cervical lymph nodes.

The external acoustic meatus is a canal that leads inward through the tympanic part of the

temporal bone from the auricle to the tympanic membrane, a distance of 2-3 cm in adults.

The lateral third of this slightly S-shaped canal is cartilaginous and lined with skin, which is

continuous with the skin of the auricle. Its medial two thirds is bony and lined with thin skin

that is continuous with the external layer of the tympanic membrane. The ceruminous and

sebaceous glands produce cerumen (earwax).

The tympanic membrane, approximately 1 cm in diameter, is a thin, oval, semitransparent

membrane at the medial end of the external acoustic meatus. It forms a partition between the

meatus and the tympanic cavity of the middle ear. The tympanic membrane is covered with

thin skin externally and the mucous membrane of the middle ear internally.

Viewed through an otoscope (an instrument used for examining the tympanic membrane), the

tympanic membrane is normally translucent and pearly gray. It has a concavity toward the

external acoustic meatus with a shallow, cone-like central depression, the peak of which is

the umbo. The handle of the malleus (one of the small ear bones, or auditory ossicles, of the

middle ear) is usually visible near the umbo. From the inferior end of the handle of the

malleus, a bright cone of light is reflected from the otoscope's illuminator. This light reflex is

visible, radiating anteroinferiorly in a healthy ear. Superior to the attachment of the lateral


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process of the malleus, the membrane is thin and is called the flaccid part (L. pars flaccida). It

lacks the radial and circular fibers present in the remainder of the tympanic membrane, called

the tense part (L. pars tensa).The tympanic membrane moves in response to air vibrationsthat pass to it through the external acoustic meatus. Movements of the membrane are

transmitted by the auditory ossicles (malleus, incus, and stapes) through the middle ear to theinternal ear . The external surface of the tympanic membrane is supplied mainly by the

auriculotemporal nerve, a branch of CN V3. Some innervation is supplied by a small auricular

branch of the vagus nerve (CN X).The internal surface of the tympanic membrane is supplied

by the glossopharyngeal nerve (CN IX).

The external auditory canal (EAC) is essentially a tube that is open at one end and closed at

the other; thus the EAC behaves like a quarter-wave resonator. The resonant frequency (f0) is

determined by the length of the tube; the curvature of the tube is irrelevant.

A flat, wide-band sound measured in a sound field is changed considerably by the acoustic

properties of the head and external ear. The acoustic properties of the external ear are one of

the reasons noise-induced hearing losses occur first and most prominently at the 4-kHzfrequency region (boilermaker notch).

In addition to the prominence of noise-induced hearing loss in the 4-kHz region, the acoustic

properties of the head and external ear have an important role in several hearing functions. In

localization of sound sources, the head acts as an attenuator at frequencies at which the width

of the head is greater than the wavelength of the sound. Thus at frequencies greater than 2

kHz, a head shadow effect occurs, in which interaural intensity differences of 5 to 15 dB are

used to localize sound sources. At lower frequencies, at which the wavelength of the sound is

larger than the width of the head, little attenuation is provided by the head. Interaural time

differences (~0.6 ms for sound to travel across the head) are the salient cues for localization.

The head-shadow effect is the reason right-handed hunters using rifles and shotguns have

larger hearing losses in their left ears than in their right ears and vice versa. The muzzle of the

gun, where the acoustic energy is greatest, is closer to the left ear, and the right ear is

protected by the head-shadow effect.

The 10- to 15-dB gain provided by the external ear in the 3- to 5-kHz region is useful for

improving the detection and recognition of low-energy, high-frequency sounds such as

voiceless fricatives. The importance of the acoustic properties of the external ear and head is

reflected in hearing-aid design and evaluations. Finally, the resonance of the external canal is

approximately 8 kHz in infants and decreases to adult values after approximately 2.5 years of

age. This developmental feature has several clinical implications, especially for sound-field

testing and for hearing-aid design and evaluation of infants.

development of the ear

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