Ear

EARserving both hearing and equilibrium. it develops from three parts: (a)the external ear, the sound-

collecting organ; (b)the middle ear, a sound conductor

from the external to the internal ear; and

(c)the internal ear, which converts sound waves into nerve impulses and registers changes in equilibrium.

INTERNAL EARIn embryos of 22 days , ear appear as a

thickening of the surface ectoderm on each side of the rhombencephalon

These thickenings, the otic placodes, invaginate rapidly and form the otic or auditory vesicles (otocysts) .

During later development, each vesicle divides into

(a) a ventral component that gives rise to the saccule and cochlear duct and

(b) a dorsal component that forms the utricle, semicircular canals, and endolymphatic duct

Together, these epithelial structures form the membranous labyrinthSaccule, Cochlea, and Organ of Corti

In the sixth week of development, the saccule forms a tubular outpocketing at its lower pole .

This outgrowth, the cochlear duct, penetrates the surrounding mesenchyme in a spiral fashion until the end of the eighth week, when it has completed 2.5 turns .

Its connection with the saccule is then confined to a narrow pathway, the ductus reuniens

Mesenchyme surrounding the cochlear duct soon differentiates into cartilage.

In the 10th week, this cartilaginous shell divided to:

,scala vestibuli and 2- scala tympani, 1. The cochlear duct is then separated from the

scala vestibuli by the vestibular membrane 2. The cochlear duct is then separated from the

scala tympani by the basilar membrane.

The lateral wall of the cochlear duct remains attached to the surrounding cartilage by the spiral ligament,

its median angle is connected to and partly supported by a long cartilaginous process, the modiolus, the future axis of the bony cochlea

epithelial cells of the cochlear duct form two ridges:

1. the inner ridge, the future spiral limbus, 2. the outer ridge3. The outer ridge forms one row of inner and three or

four rows of outer hair cells, the sensory cells of the auditory system.

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They are covered by the tectorial membraneThe sensory cells and tectorial membrane

together constitute the organ of Corti.

Impulses transmitted to the spiral ganglion and then to the nervous system by the auditory fibers of cranial nerve VIII.

Utricle and Semicircular CanalsDuring the sixth week of development, semicircular

canals appear as flattened outpocketings of the otic vesicle.. Central portions disappear, giving rise to three semicircular

canalsWhereas one end of each canal dilates to form the crus

ampullare, the other, the crus nonampullare

crista ampullaris, containing sensory cells for maintenance of equilibrium.

maculae acusticae, develop in the walls of the utricle and saccule.

statoacoustic ganglion formed from cells from otic vesicle & from

neural crest ,it splits into1. cochlear portion supply the organ of Corti 2. vestibular portions, the saccule, utricle,

and semicircular canals

MIDDLE Ear :Tympanic Cavity and Auditory TubeThe tympanic cavity, which originates in the

endoderm, is derived from the first pharyngeal pouch .

The distal part of the pouch, the tubotympanic recess, widens and gives rise to the primitive tympanic cavity, and the proximal part remains narrow and forms the auditory tube (eustachian tube) ,through which the tympanic cavity communicates with the nasopharynx.

OssiclesThe malleus and incus are derived from cartilage

of the first pharyngeal arch, and the stapes is derived from that of the second

arch they remain embedded in mesenchyme until the

eighth month The supporting ligaments of the ossicles develop later.

During late fetal life, the tympanic cavity will form the tympanic antrum.

After birth, the epithelium of the tympanic cavity invades the bone of the developing mastoid process, and epithelium-lined air sacs are formed (pneumatization).

Later, most of the mastoid air sacs come in contact with the antrum and tympanic cavity.

Expansion of inflammations of the middle ear into the antrum and mastoid air cells is a common complication of middle ear infections

EXTERNAL EARExternal Auditory Meatusdevelops from the

dorsal portion of the first pharyngeal cleft.

At the beginning of the third month, forming a solid epithelial plate, the meatal plug

In the seventh month, this plug dissolves,.

Occasionally, the meatal plug persists until birth, resulting in congenital deafness.

The eardrum( tympanic membrane) is made up of (a) an ectodermal epithelial lining of the auditory

meatus, (b) an endodermal epithelial lining of the tympanic

cavit(c) an intermediate layer of connective tissue that

forms the fibrous stratum. The major part of the eardrum is firmly attached to

the handle of the malleus .and the remaining portion forms the separation between the external auditory meatus and the tympanic cavity.

AuricleThe auricle develops from six mesenchymal proliferations

at the dorsal ends of the first and second pharyngeal arches, surrounding the first pharyngeal cleft

These swellings (auricular hillocks), three on each side of the external meatus, later fuse and form the definitive auricle

As fusion of the auricular hillocks is complicated, developmental abnormalities of the auricle are common.

Clinical Correlates Deafness and External Ear AbnormalitiesCongenital deafness, usually associated with deaf-mutism, may be caused by1. abnormal development of the membranous and bony

labyrinths2. malformations of the auditory ossicles and eardrum. In the most extreme cases, the tympanic cavity and

external meatus are absent.Most forms of congenital deafness are caused by 3. genetic factors,mostly4. environmental factors

1. Rubella virus, 2. poliomyelitis,3. erythroblastosis fetalis4. diabetes5. hypothyroidism, 6. toxoplasmosis

External ear defects 1. They causes psychological and emotional

trauma 2. they are often associated with other

malformations. Thus, they serve as clues to examine infants carefully for other abnormalities.

Preauricular appendages and pits

skin tags: are appendages anterior to the ear. may be caused by accessory hillocks.

Pits shallow depressions may indicate abnormal development of the auricular hillocks,

Like other external ear defects, both are associated with other malformations.

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