Inner Ear Anatomy : Embryology and Parts

The ear is one of the most complex parts of our body and to make the study of ear anatomy possible it is divided into 3 parts. The parts of the ear are the external ear, middle ear, and inner ear. The inner ear is made up of Bony Labyrinth and membranous labyrinth. Bony labyrinth is the outer covering and membranous labyrinth is present within Bony labyrinth.

Embryology of Inner Ear

• 1^st and 2^nd pharyngeal arch forms 6 tubercles known as hillocks of his. First hillocks give rise to tragus and other hillocks fused together to form Pinna.
• The cleft of first pharyngeal arch forms external auditory canal.
• The pouches of 1^st and 2^nd pharyngeal arches fuse to form tubo-tympanic recess which further forms eustachian tube and middle ear.
• There is a thickened ectodermal patch called as otic placode which gets converted into otic vessicle and which further leads to formation of inner ear.
• Among the Bony ossicles, malleus and incus are mesodermal and these both are formed from first pharyngeal arch. The suprastructure of stapes is developed from second pharyngeal arch but the footplate of stapes is formed from otic capsule. The stapes is also mesodermal in origin.

Also Read: Laryngeal Trauma and Laryngotracheal Stenosis

Parts of Inner Ear

The labyrinth is divided into 3 parts: vestibule, cochlea, and semicircular canals.

• Vestibule has two parts named utricle and saccule.
• The vestibule is the central chamber and it has two Bony recesses or depressions.
• One Bony depression is called an elliptical recess and it contains a utricle that is posteriorly connected to the opening of the semicircular canal duct. The other Bony depression is called a spherical recess and it contains saccule which is anteriorly connected to the cochlear duct via ductus reuniens.
• The utricle lies in the posterior portion of the Bony vestibule and it has a sensory epithelium known as a macula whose function is to manage linear acceleration and deceleration.
•  The saccule is anterior to the utricle and opposite to the stapes footplate. Its sensory epithelium is also called as macula and its exact function is not known.
• The cochlea constitutes of cochlear duct. It is a spiral-shaped structure that resembles a snail and it rotates around the central axis known as modiolus.
  • It has a cochlear duct or cochlear chamber which is covered by a Bony covering. The cochlear duct is divided into the scala vestibuli above, the scalar media in the middle, and the scalar tympani below.
  • At the basal part, the Oval window is connected to the scala vestibuli on one end and the scala tympani on the other end. Scala tympani is covered by a round window.
  • The scala vestibuli communicates with the scala tympani and this is known as helicotrema. Striae vascularis is present on the lateral wall of scala media and it is responsible for production and resorption of endolymph.
  • The reissners membrane separates scala vestibuli and scala media. The basilar membrane separates scala media from scala tympani.
  • The organ of corti is a neurosensory epithelium responsible for hearing. It is present on the basilar membrane in scalar media.
  • The organ of corti has hair cells that act as receptors for hearing and it converts sound energy into electrical energy. There are two types of hair cells, those are inner hair cells and outer hair cells.
  • There is one more communication and it is called as Aqueduct of cochlea. It is a communication between scala tympani and sub-arachnoid space.
  • The perilymph reaches scala tympani and scala vestibuli via the aqueduct of cochlea and it flows between the bony and membranous labyrinth.
  • Emi-circular canals are at 90° to each other and they are mainly responsible for maintaining the balance in all three axes.
  • Crista ampularis is a neurosensory epithelium in the ampullated end of semicircular canals and it is mainly responsible for angular or rotational motion.

• The endolymphatic duct is formed by the union of the utricle duct and saccular duct.
• The perilymph is present between Bony and membranous labyrinth and it is rich in sodium. Endolymph fills the membranous labyrinth, it is produced and reabsorbed by striae vascularis and it is rich in potassium.
• The endolymph flows from cochlear duct to endolymphatic duct via saccule. From endolymphatic duct it reaches endolymphatic sac and then it flows towards semicircular canals via utricle.

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