Inner Ear Anatomy : Embryology and Parts

The ear is one of the most complex parts of our body. This blog will discuss the parts of the ear, the embryology of the inner ear, and some frequently asked questions related to these topics to facilitate the study of ear anatomy.

Parts of ear

the ear is divided into three parts

• The external ear
• The middle ear
• The inner ear.
  • The inner ear consists of a Bony Labyrinth and a membranous labyrinth. 
  • The bony labyrinth is the outer covering, and the membranous labyrinth is present within it.

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 the first pharyngeal arch forms the external auditory canal.
• The pouches of the first and second pharyngeal arches fuse to form a tubo-tympanic recess, which further forms the eustachian tube and middle ear.
• A thickened ectodermal patch called the otic placode gets converted into an otic vesicle and further forms the inner ear.
• Among the Bony ossicles, the malleus and incus are mesodermal, and both are formed from the first pharyngeal arch. The stapes's superstructure is developed from the second pharyngeal arch, but the footplate is formed from the otic capsule. The stapes are 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 the Bony and membranous labyrinth and is rich in sodium. Endolymph fills the membranous labyrinth; it is produced and reabsorbed by the striae vascularis and is rich in potassium.
• The endolymph flows from the cochlear duct to the endolymphatic duct via the saccule. From the endolymphatic duct, it reaches the endolymphatic sac, where it flows towards the semicircular canals via the utricle.

Frequently Asked Questions

Q: Which is the membrane that separates scala vestibuli and scala media?

Answer: Reissner’s membrane.

Q: What part of the ear contains the three bones?

Answer: Middle Ear

Q: Which cartilage forms the external part of the ear?

Answer: Hyline Cartilage.

Q: What are the parts of the ear?

Answer: The outer, middle, and inner ear.

Q: What are the two ducts that form the endolymphatic duct?

Answer: The endolymphatic duct is formed by the union of the utricle duct and saccular duct

Hope you found this blog helpful for your ENT residency Larynx preparation. For more informative and interesting posts like these, keep reading PrepLadder’s blogs.