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Embryology of Ear

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Embryology of the Ear

The development of the ear is a complex process that begins early in the embryonic stage and involves multiple structures derived from the pharyngeal arches. Understanding the embryology of the ear requires a basic understanding of these pharyngeal arches and how they contribute to the formation of different parts of the ear.

Pharyngeal Arches and Germinal Layers

Six pairs of well-defined outgrowths called pharyngeal arches develop at week 4, lateral to the stomodeum and above the cardiac bulge. These pharyngeal arches are a proliferation of mesoderm germ layer infiltrated by the neural crest cells. The fifth arch appears briefly but disappears soon after its formation. Each pharyngeal arch is composed of three germinal layers:

  1. Endoderm: Forms the inner lining of the pharyngeal arches.
  2. Ectoderm: Forms the outer surface of the arches.
  3. Mesoderm: Located between the endoderm and ectoderm, it forms the structural framework of the arches.

Each arch has its own blood supply and nerve supply. Between the two pharyngeal arches, there are clefts on the ectodermal (outer) side and pouches on the endodermal (inner) side. These clefts and pouches play key roles in forming various structures of the ear.

1. Development of Pinna

It is formed by six small prominences/ tubercles called hillocks of His around the 6th week of embryonic life. These hillocks develop from the first and second pharyngeal arches (around 1st cleft). The first arch forms the first hillock, while the remaining hillocks come from the second arch. The first hillock specifically forms the tragus, while the rest of the pinna develops from the second hillock. The incisura terminalis marks the line of separation between the first and second hillocks. These hillocks fuse together to form the pinna. Interestingly, an improper fusion of these hillocks can lead to conditions such as preauricular sinuses or cysts, often located between the tragus and the helix (at the root of the helix). By the 20th week, the auricle achieves its adult shape and migrates to a more lateral and cranial position.

 

2. Development of External auditory canal, Tympanic membrane and Eustachian tube.

The first pharyngeal pouch invaginates inward to form the Eustachian tube, while the first pharyngeal cleft invaginates inward to form the external auditory canal. Where the pouch and cleft meet is where the tympanic membrane (eardrum) forms. The tympanic membrane is made up of all three germinal layers: ectoderm, mesoderm, and endoderm.

  • By the 16th week, cell proliferate from the bottom of the ectodermal cleft forms a meatal plug, which recanalizes to create the ear canal starting from the bony meatus near the tympanic membrane and progressing outward. That is why deeper meatus is sometimes developed while there is atresia of the canal in the outer part.
  • The external ear canal is fully developed by the 28th week of gestation.

3. Development of Middle ear cleft.

The middle ear cleft—comprising the Eustachian tube, tympanic cavity, attic, antrum, and mastoid air cells—develops from the endoderm of the tubotympanic recess, which arises from the first and partly from the second pharyngeal pouches. The ossicles (malleus, incus, and stapes) also take shape during this time. The malleus and incus derived from the first arch, and the stapes, except for its footplate and annular ligament, from the second arch. The footplate and annular ligament develop from the otic capsule. The middle ear is completely developed at the time of birth. The eustachian tube and middle ear is formed by 8 months gestation and the epitympanum and mastoid antrum are developed by birth.

4. Development of Inner ear. The inner ear development begins in the 3rd week of foetal life and is completed by the 16th week.

(i) Membranous labyrinth. The development of the inner ear begins with the formation of a thickened area of external ectoderm epithelium known as the otic placode. As the embryo grows, this placode starts to invaginate (fold inward), forming the otic pit. Eventually, the otic pit separates from the external epithelium and forms a distinct structure called the otic vesicle (otocyst).

The otic vesicle undergoes further differentiation into two parts which are derived from the ectoderm:

  • Dorsal utricular portion (Pars superior): Forms the vestibular system, which includes the utricle, semicircular canals, and endolymphatic tube. It develops earlier than pars inferior.
  • Ventral saccular portion (Pars inferior): Forms the hearing system, including the cochlear ducts and saccule.

By the 20th week, the cochlea (organ of corti) is fully developed, and the foetus is capable of hearing sounds within the womb. This is often referenced in historical and cultural anecdotes, such as the story of Abhimanyu from the Mahabharata, where the unborn child hears conversations in the womb.

(ii) Bony labyrinth. Surrounding the otic vesicle is mesoderm, which thickens to form the otic capsule. The otic capsule later differentiates into the cartilaginous framework of the inner ear, which eventually ossifies to become the bony labyrinth. This cartilage also ossifies into the petrous part of temporal bone, which encloses and protects the delicate structures of the internal ear.

Summary of Ear Development

  • External Ear:
    • Pinna: Formed by six hillocks of His (from the first and second arches).
    • External Auditory Canal: Formed by the first pharyngeal cleft.
    • Tympanic Membrane: Formed by all three germinal layers (ectoderm, mesoderm, endoderm).
  • Middle Ear:
    • Eustachian Tube: Formed by the first pharyngeal pouch.
    • Tympanic cavity, attic, antrum, and mastoid air cells – Formed by the first and partly from the second pharyngeal pouches.
    • Ossicles
      • Malleus and Incus – Derived from the first arch.
      • Stapes – Derived from the second arch.
      • Footplate and annular ligament – Formed by the otic capsule.
  • Inner Ear:
    • Membranous Labyrinth: Derived from the otic vesicle (ectoderm).
    • Bony Labyrinth: Formed by the otic capsule (mesoderm).

The development of the outer, middle, and inner ear occurs independently. As a result, malformations in one part of the ear do not necessarily affect the other parts, allowing for normal function in some regions despite abnormalities in others.

———— End of the chapter ————

Download full PDF Link:
Embryology of Ear Best Lecture Notes Dr Rahul Bagla ENT Textbook

Reference Textbooks.

  • Scott-Brown, Textbook of Otorhinolaryngology-Head and Neck Surgery.
  • Glasscock-Shambaugh, Textbook of Surgery of the Ear.
  • P L Dhingra, Textbook of Diseases of Ear, Nose and Throat.
  • Hazarika P, Textbook of Ear Nose Throat And Head Neck Surgery Clinical Practical.
  • Mohan Bansal, Textbook of Diseases of Ear, Nose and Throat Head and Neck Surgery
  • Hans Behrbohm, Textbook of Ear, Nose, and Throat Diseases With Head and Neck Surgery.
  • Salah Mansour, Middle Ear Diseases – Advances in Diagnosis and Management.
  • Logan Turner, Textbook of Diseases of The Nose, Throat and Ear Head And Neck Surgery.
  • Rob and smith, Textbook of Operative surgery.
  • Anirban Biswas, Textbook of Clinical Audio-vestibulometry.
  • Arnold, U. Ganzer, Textbook of  Otorhinolaryngology, Head and Neck Surgery.

Author:

Dr. Rahul Bagla ENT Textbook

Dr. Rahul Bagla
MBBS (MAMC, Delhi) MS ENT (UCMS, Delhi)
Fellow Rhinoplasty & Facial Plastic Surgery.
Renowned Teaching Faculty
Mail: msrahulbagla@gmail.com
India

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