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| Main Anatomy Index | Ear
Last updated 30 March 2006
This page was contributed by David
Development of the membranous labyrinth
The endolymphatic sac at the distal end of the endolymphatic duct
- In the 3rd week, a pair of ectodermal thickenings, the otic
placodes, develop on the sides of the rhombencephalon (hindbrain), later
shifting caudally to the level of the 2nd pharyngeal arch.
- In the 4th week, each otic placode invaginates, forming an otic
pit, which pinches off from the surface ectoderm to form the otic
- The otic vesicle, precursor of the definitive membranous labyrinth,
differentiates into 3 parts:
- A dorsomedial, elongated endolymphatic appendage, which
A central, expanded utricular region, which will form:
- The utricle
- The 3 semicircular ducts, which arise from utricular diverticula that appear in the 7th week.
- A ventral, conical saccular region, which forms:
- The saccule
- The cochlear duct, which coils to about 2.5 spirals and elongates in the 5th week, with the
spiral organ of Corti differentiating in the 7th
- The ductus reuniens joining the saccule and cochlear
- The organ of Corti is innervated by the spiral ganglion of the modiolus, whose fibres form the cochlear branch of the vestibulocochlear
nerve (CN VIII), and synapse in the medial geniculate bodies of the brain.
- The maculae of the utricle and saccule and the ampullae of the semicircular ducts are innervated by the vestibular ganglion, whose fibres form the vestibular
branch of the vestibulocochlear nerve.
Development of the bony labyrinth
- In the 9th week, the mesenchyme around the membranous labyrinth is induced by
the otic vesicle to chondrify, forming the cartilagenous otic capsule, which undergoes vacuolisation
to produce the perilymphatic cavity surrounding the
- The membranous labyrinth becomes suspended in the perilymph
of this cavity, and the otic capsule eventually ossifies by 23 weeks, forming the bony labyrinth of the petrous part of the temporal bone.
Development of the Tympanic cavity
- The 1st pharyngeal pouch elongates to form
the tubotympanic recess, which will give rise to the tympanic cavity and the auditory tube.
- In the 7th week, the ossicles begin to
condense from the mesenchyme of the 1st and 2nd pharyngeal arches,
whilst the middle ear muscles begin forming in the 9th
derivatives in the middle ear
||Middle ear structures
- Cartilage ---> malleus, incus
- Mesoderm ---> tensor tympani
- Cartilage ---> stapes
- Mesoderm ---> stapedius
pharyngeal arch derivatives
|Pharyngeal arch structure
||Middle ear structures
|1st pharyngeal cleft
||External acoustic meatus
|1st pharyngeal pouch
- The tympanic membrane has 3 layers:
A mesodermal layer called the fibrous stratum
An inner lining of endoderm
- An outer lining of ectoderm
- In the 9th month, the ossicles assume their functional relationships, with
the malleus attaching to the eardrum, and the stapes attaching to the oval window.
- Sound vibrations can now be transmitted from the
eardrum to the cochlea via the ossicles and oval window, and then transduced
into neural impulses via the organ of Corti.
Development of the external ear
- The auricle develops from 3 pairs of auricular
hillocks that arise in the 5th week, on the lateral faces of the 1st
and 2nd pharyngeal arches.
- The hillocks are named for the portions of the auricles
to which they give rise.
- In the 7th week, the auricular hillocks
begin to enlarge, differentiate and fuse, producing the final shape of the ear, which is
gradually translocated from the side of the neck to a
more cranial and lateral site.
of the auricle
Hillocks ---> Resulting part of pinna
(from ventral to dorsal on pharyngeal arch)
- Cymba concha