Embryology of the Ear
Development of the ear begins at 3 weeks of gestation and is usually complete by 20 weeks of gestation.
Inner ear
- Ectoderm overlying the rhombencephalon thickens to form the otic placode.
- The otic placode invaginates to become the otic pit.
- The otic pit detaches from surface ectoderm to form the otic vesicle.
- The otic vesicle divides into dorsal and ventral compartments.
- Dorsal compartment gives rise to:
- Semicircular canals
- Utricle
- Endolymphatic appendage/sac
- Ventral compartment gives rise to:
- Cochlea
- Spiral of the cochlea
- Dorsal compartment gives rise to:
- Otic vesicle is filled with endolymph, with surrounding mesenchyme that hollows out to create spaces filled with perilymph.
- Perilymph spaces become the scala tympani and scala vestibuli.
- Neural crest–derived cells → statoacoustic ganglion → cranial nerve (CN) VIII
Transverse section of the embryo featuring the otic pit becoming the otic vesicle, which migrates into the mesenchyme
Image by Lecturio.
Transverse section of the embryo, featuring the otic placode originating from the ectoderm and invaginating to become the otic pit
Image by Lecturio.
Diagram displaying the development of the otic vesicle into 2 compartments: dorsal and ventral:
Image by Lecturio.
Note that the dorsal compartment has an endolymphatic sac that extends toward the brain. The ossicle condensations are featured in brown, while the auditory tube is shown in yellow.
Middle ear
- Develops from the 3 germinal layers
- Auditory tube extends from the 1st pharyngeal pouch.
- 1st and 2nd pharyngeal arches give rise to the malleus, incus, and stapes.
- Auditory tube surrounds the ossicles → tympanic cavity
- Tympanic membrane
- Inner layer (endoderm)
- Middle layer (mesoderm)
- Outer layer (ectoderm)
Diagram displaying the auditory tube, which extends from the 1st pharyngeal pouch as it surrounds the ossicles, creating the tympanic cavity
Image by Lecturio.Outer ear
- Groove between 1st and 2nd pharyngeal arches → external acoustic meatus
- Pharyngeal arches → auricular hillocks → external ear/auricle
The 6 auricular hillocks in the fetus bulge outward from the pharyngeal arches and shift to form the characteristic shape of the external ear.
Image by Lecturio.Outer Ear Anatomy
Auricle
The auricle is composed of musculocutaneous tissue and elastic cartilage covered by skin. This structure is responsible for collecting and redirecting sound waves into the external auditory meatus.
- Helix: curved outer rim
- Antihelix: inner curved rim
- Tragus and antitragus: protrusions that partially obstruct external auditory meatus
- Concha: hollow region in front of external auditory meatus
- Lobule: inferior, fleshy portion of ear
- Triangular fossa: concave surface of anterosuperior ear
- Scaphoid fossa: concave surface of posterosuperior ear
The outer ear, featuring the anatomical landmarks of the auricle
Image by Lecturio.External auditory meatus
The external auditory meatus is typically 1 in. (2.5 cm) in length and is S-shaped.
- Lateral ⅓ surrounded by cartilage (posterosuperior trajectory)
- Medial ⅔ surrounded by temporal bone (anteroinferior trajectory)
- Surrounding skin contains ceruminous and sebaceous glands → ear wax
Tympanic membrane
Transmits sound waves from the air to the ossicles → mechanical vibrations
- Located between outer and middle ear
- Attached to fibrocartilaginous ring of temporal bone
- 3 layers: cutaneous, fibrous, and mucous
- Ossicles attach to the tympanic membrane at the umbo.
Tympanic membrane:
Pars flaccida (superior to insertion manubrium) and pars tensa (remainder of tympanic membrane)
Middle Ear
Tympanic cavity
- Bony membranous cavity shaped like a 6-sided prism and filled with air
- Contains the ossicles, muscles, and nerves
- The roof consists of the tegmental wall, which separates the malleus and incus from the middle cranial fossa.
- Labyrinthine or medial wall contents:
- Oval window (connects to vestibule of the internal ear)
- Round window (secondary tympanic membrane)
- Promontory
- Prominence of the facial canal
- Relations:
- Lateral wall: tympanic membrane
- Posterior wall:
- Mastoid antrum
- Communicates with mastoid air cells
- Anterior wall (carotid wall):
- Thin layer of bone separating the cavity from the carotid artery
- Pierced by the tympanic branch of the internal carotid artery and deep petrosal nerve
- Contains the auditory tube
- Floor (jugular wall): separates the cavity from the internal jugular vein
- Auditory ossicles:
- Amplify sound waves and transmit them to the labyrinth of the inner ear
- Connect the tympanic membrane to the oval window
- Smallest bones in the human body
- Lateral → medial: malleus, incus, stapes
The middle ear, featuring the auditory ossicles connecting the tympanic membrane to the oval window
Image: “Anatomy of the middle ear” by BruceBlaus. License: CC BY 3.0Muscles of the middle ear
Middle ear muscles are responsible for movement of the tympanic membrane.
- Tensor tympani muscle:
- Origin:
- Cartilaginous part of eustachian tube
- Greater wing of the sphenoid
- Semicanal for tensor tympani (petrous part of temporal bone)
- Insertion: superior part of the handle of the malleus
- Innervation: mandibular branch of the trigeminal nerve (CN V3)
- Function:
- Pulls the handle of the malleus medially
- Tenses the tympanic membrane
- Origin:
- Stapedius muscle:
- Origin: cartilaginous/bony margins of eustachian tube
- Insertion: neck of stapes
- Innervation: facial nerve (CN VII)
- Function:
- Pulls stapes posteriorly
- Prevents excessive oscillation of the ossicles
Muscles and ossicles of the middle ear:
The malleus connects to the tympanic membrane while the incus and stapes are completely within the middle ear. The stapedius muscle prevents excessive oscillation of the ossicles.
Eustachian tube (auditory tube)
- Controls pressure within the middle ear and on both sides of the tympanic membrane
- Located anterior to the tympanic cavity
- Connects the middle ear to the nasopharynx
The Eustachian tube connects the middle ear with the pharynx:
This connection allows for the equalization of air pressure between the middle ear and the outside world.
Inner Ear
The inner ear is located within the petrous part of the temporal bone and contains the organs of hearing and equilibrium.
Bony labyrinth
The bony labyrinth is filled with perilymph and is separated from the middle ear by the medial wall of the tympanic cavity.
- Vestibule: central bony cavity:
- Contains the utricle and saccule
- Communicates with the middle ear in 3 directions:
- Laterally through the oval window
- Anteriorly with the cochlea
- Posteriorly with the semicircular canals
- Semicircular canals: 3 canals at 90-degree angles to each other:
- Anterior canal = sagittal plane
- Posterior canal = frontal plane
- Lateral canal = transverse plane
- Cochlea: spiral-shaped bony cavity:
- Spiral lamina: superior scala vestibuli and inferior scala tympani
- Central bony column = modiolus
Diagram of the bony and membranous labyrinths, featuring their anatomical landmarks:
Note that the cochlear duct is found within the membranous labyrinth and the utricle and saccule are found within the vestibule.
Membranous labyrinth
The membranous labyrinth is filled with endolymph and is composed of the vestibular and cochlear labyrinths.
- Vestibular labyrinth:
- Comprised of the utricle, saccule, and 3 membranous semicircular ducts
- Contains sensory organs for balance
- Cochlear labyrinth:
- Sensory organ for hearing
- Scala vestibuli:
- Contains perilymph
- Anterior chamber up which sound travels
- Scala tympani:
- Contains perilymph
- Posterior chamber down which sound travels
- Scala media (cochlear duct):
- Contains endolymph
- Located between scala vestibuli and scala tympani
- Houses organ of Corti
- Organ of Corti:
- Spiral organ within cochlear labyrinth
- Contains mechanoreceptors called hair cells
- Hair cells determine the movement and position of the head in space.
- Types I (bulbous) and II (elongated) hair cells
- Both types have stereocilia of varying lengths at the apex.
- Stereocilia respond to movement of endolymph.
- Kinocilium: single cilium at lateral portion of hair cell, defines polarity of the cell
- Hair cells in the organ of Corti lack kinocilium.
The organ of Corti is located within the scala media:
The organ of Corti detects sound in the form of vibration and transmits the information through the cochlear nerve.
CN: cranial nerve
Types I and II hair cells:
Hair cells determine the movement and position of the head in space.
Neurovasculature of the Ear
| Structure | Blood supply | Lymphatics | Innervation |
|---|---|---|---|
| Auricle |
| Superficial parotid, mastoid, and cervical lymph nodes |
|
| Tympanic membrane |
| Periauricular lymph nodes |
|
| Tympanic cavity |
| Retroauricular cervical lymph nodes |
|
| Bony labyrinth |
| Parotid, mastoid, and superficial cervical lymph nodes |
|
| Membranous labyrinth | Labyrinthine artery (branch of the basilar artery) |
Clinical Relevance
- Otitis externa: inflammation of the external acoustic meatus. Otitis externa is a bacterial infection that most commonly occurs in children. The typical causative organisms are Pseudomonas and Staphylococcus aureus. Presentation is with exquisite tenderness on manipulation of the external ear. Treatment is with antibiotic eardrops.
- Acute otitis media: acute inflammation of the middle ear due to viral or bacterial infections. The condition is most common in children ages 6–18 months. Presentation is with fever, symptoms of upper respiratory tract infection, and ear pain. The majority of cases are caused by viruses, but antibiotics should be initiated if a bacterial etiology is suspected.
- Vertigo: sensation of movement between oneself and surroundings. Vertigo is not limited to a feeling of rotation but may include other forms of movement, such as upward lifting, swaying, rocking, or nonsystematic movement. It is usually a symptom of an underlying condition such as labyrinthitis, Ménière disease, or an adverse reaction to medications.
- Ménière disease: characterized by a symptom triad of vertigo, tinnitus, and hearing impairment. The episodes of vertigo in Ménière disease can last from a few minutes to several hours, and the hearing impairment mostly affects low frequencies. Treatment is geared toward nausea prevention and physical therapy to maintain physical functionality.
- Hearing loss: hearing impairments are classified into conductive hearing loss and sensorineural hearing loss. Conductive hearing loss stems from damage to the middle ear, which prevents the proper transmission of sound. In sensorineural hearing loss, the hearing organ is damaged.
- Tinnitus: perception of noise or “ringing in the ears.” Tinnitus is a common condition that is experienced in the absence of an external auditory stimulus. The sound perceived is described as whistling, hissing, buzzing, ringing, and/or pulsating. Tinnitus may be related to underlying conditions such as Ménière disease, ear and sinus infections, and brain tumors.
References
- Bhatt RA. (2020). Ear anatomy: overview, embryology, gross anatomy. Medscape. https://emedicine.medscape.com/article/1948907-overview
- ONeill OJ, Brett K, Frank AJ. (2021). Middle ear barotrauma. StatPearls. https://www.ncbi.nlm.nih.gov/books/NBK499851/
- Szymanski, A. Geiger, Z. (2017). Anatomy, head and neck, ear. StatPearls. https://europepmc.org/article/nbk/nbk470359
