Development of ear
Learning Objectives
At the end of lecture student will be able to:
Describe the development of ear.
Differentiate between development of internal, middle and external ear.
Enlist the common disease associated with the development of ear.
Ear Gross Anatomy
Development of Ear
The ear is composed of three parts:
External ear ---pinna, external acoustic meatus& tympanic membrane.
Middle ear ---three small ossicles, internal layer of tympanic membrane.
Internal ear— vestibulocochlear organ– concerned with balance and hearing.
Ear Gross Anatomy
Internal Ear
Development of Internal Ear
Internal ear is 1st of three part to develop at the early 4th week as the thickening of surface ectoderm the otic placode on each side of myelencephalon.
Each otic placode soon invaginates and sinks into the underlying mesnchyme otic pits.
The edges of otic pits soon come together to fuse to form otic vesicle the primordium of membranous labyrinth.
Otic vesicle soon loose its connect from the surface ectoderm.
Development of Internal Ear
A diverticulum grows from the vesicle to form endolymphatic duct and sac.
Now 2 regions are recognizable:
- A dorsal utricle part - semicircular ducts arises from it.
- A ventral saccular part –saccule and cochlear duct.
Development of Internal Ear
From the utricular part 3 diverticula grow:
Soon the central part of these diverticula fuse and disappeare –peripheral parts become semicricular ducts—later enclose by the bony labyrinth.
Localize dilatation in the canal—ampullae develop receptor part the cristea ampullares.
Development of Internal Ear
From ventral saccular part the otic vesicle — a tubular diverticulum –the cochlear duct—grows and coils to form the membranous cochlea.
Connection of the cochlea with saccule—the ductus reuniens soon forms.
Spiral ganglion cells of vestibulocochlear nerve migrates along the coils of membranous labybrinth to form spiral ganglion.
As the memranous labyrinth enlarges ,vacuoles appears in the cartilaginous otic capsule—
There is perilymphatic space—mebranous labyrinth is suspended in the perlymph fliud.
The perlymphatic space related to the cochlear ducts develops into scala tympani and scala vetibuli.
The cartilaginous otic capsule later ossifies into bony labyrinth of internal ears.
Development of Middle ear
Development of tubo-tympanic recess from the 1st pharyngeal pouch.
Proximal part of tubotympanic recess form the (auditory tube).
The distal part of the recess expand the tympanic cavity , which gradually envelopes the small bones of middle ear, their tendons and ligaments and chorda tympani nerve.
Development of Middle ear
These structure receive more or less complete epithelial investment.
During the late fetal period expansion of tympanic cavity gives—mastoid antrum.
Development of External Ear
The external acoustic meatus—passage of the external ear to the tympanic membrane –develops from the dorsal part of the first pharyngeal groove.
The ectodermal cell at the bottom of funnel shaped tube proliferate to form a solid epithelial plate—meatal plug.
Development of External Ear
Late in the fetal period , central cells of this plug degenerates –forming cavity of internal part of external acoustic meatus.
External acoustic meatus is short at birth attains its adult length at ninth years.
Tympanic Membrane
The promodium of tympanic membrane is the 1st phyrangeal membrane, external surface of tympanic membrane.
The phyrangeal membrane separates the 1st phyrangeal groove from 1st phyrangeal pouch.
Tympanic Membrane
The tympanic membrane develops from:
Ectoderm of first pharyngeal groove.
Endoderm of tubotympanic recess
Mescenchyme of 1st and 2nd phryngeal arches
Development of Auricle
Aurilce –projects from the side of head, develops from the mesenchymal proliferation in the 1st and 2nd phryngeal arches—auricular hillocks—surrounding the 1st groove.
Auricular grows by the contribution of the 1st arch.
Development of Auricle
The lobule is the last part of auricle to develop.
The auricle begins to develops at the base of neck—as the mandible develops—the auricle assume their normal position at the side of head.
Clinical Correlation
Congenital deafness due to rubella infection.
Microtia.
Absence of auricle.
Preauricular sinus and fistula.
Microtia.