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Embryology of the Ear

EAR DEVELOPMENT

Ear has a very complex source of development. The sound conductive apparatus develops from the branchial apparatus whereas the sound perceptive apparatus develops from the ectodermal otocyst (pars otica). Because of this dual source of origin the developmental anomaly that produced commonly affects either the sound conductive system which includes anomaly of the external and/or the middle ear or the sound perceptive apparatus which includes the labyrinth. Both these anomalies rarely coexist because of different source of origin.
Development of the External Ear
This develops around the first branchial cleft.
The Pinna
Around 6th week of intrauterine life six hillocks or ‘tubercles of His’ appear around the first branchial cleft. The first tubercle is derived from the first branchial arch and the rest from the 2nd branchial arch. Some authors believe that the first 3 tubercles develop from the first arch and the rest from the 2nd arch.
Structures derived from Various Hillocks
1. Tragus
2. Crus of the helix
3. Helix
4. Antihelix
5. Scapha and the antitragus
6. Earlobule
The ear takes definitive form by the end of third month of intrauterine life. Defective fusion of the tubercles gives rise to preauricular sinus and failure of the development of the hillocks causes anotia. Defective development of 4th tubercle can cause absence of antihelix leading to ‘bat ear’ deformity.

External Auditory Canal
This develops around the first branchial cleft as an invagination into a funnel-shaped pit to form a primary external auditory canal. Subsequent medial growth with a solid core of ectoderm leads to formation of a meatal plate called the secondary external auditory canal. Between 8th and 10th week of IUL, the solid core of epithelium undergoes canalization forming the definitive external auditory canal.
Anomalies of the External Auditory Canal
1. Complete atresia
2. Shallow depression
3. Changes in the curvature of the canal
4. Stenosis
Development of Tympanomastoid Cavity and Eustachian Tube
Around 3rd week of IUL the first pharyngeal pouch develops which is phylogenetic ally the aquatic gill apparatus. This out pouching of the first pharyngeal pouch gives rise to two components namely:
1. The proximal narrow part which forms the eustachian tube.
2. The distal dilated part which gives rise to the developing middle ear cleft and is known as the tubotympanic recess. This forms the definitive tympanic cavity by progressively and systematically invaginating into the adjacent mesenchyme.
Towards the later part of the fetal life a diverticulum appears from the tubotympanic recess which subsequently forms the mastoid antrum. This antrum is about 3 mm thickness at birth and it increases 1 mm every year till it reaches the adult size of 15 mm thickness.

Development of Ossicles
Anson in 1959 described the details of the development of the ossicles. The first arch cartilage (Meckel’s cartilage) forms the head of the malleus and the body of the incus. The second arch forms the manubrium (handle) of the malleus and the long process of the incus and the crurae of the stapes. These sources of development confirm the various developmental anomalies involving the ossicles as encountered during surgery. The foot plate of the stapes develops from three sources namely:
1. The outer periosteal layer of the otic capsule.
2. Middle enchondral layer from the otic capsule.
3. Inner endosteal layer is same as the endosteum of the bony labyrinth and develops from the periotic mesoderm.

Development of Middle Ear Spaces and Folds
The envelopment of the ossicles by the mucous membrane lining of the tubotympanic recess occurs between 3-7 months. This mucous lining while encircling the ossicles form numerous folds and spaces as follows

COMPARTMENT AND FOLDS OF THE TYMPANIC CAVITY
1. Attic compartments
2. Compartment of mesotympanum
Anterior malleolar fold
• Neck of malleus to ant. margin of tympanic sulcus.
Posterior malleolar fold
• Neck of malleus to post margin of tympanic sulcus.
Tensor Tympani fold
Tensor tympani tendon

Prussak’s Space
It is a potential space which may be the first to involve during the extension of cholesteatoma, it is bounded by:
• Laterally by shrapnell’s membrane (Pars flacida)
• Medially by the neck of malleus
• Superiorly by fibers of lateral malleolar fold
• Inferiorly lateral process of malleus

Development of the Inner Ear
The inner ear develops from the otic capsule (pars otica). Initially a thickening appears in the ectoderm of the hindbrain known as otic placode. It later invaginates forming otic cyst which is also known as otic capsule. Subsequent differentiation of this otic cyst leads to formation of membranous labyrinth. The mesoderm surrounding the otic capsule forms the bony labyrinth which attains the adult size at around 4th week of fetal life.

POINTS TO REMEMBER

1. The sound conducting apparatus develops from the branchial apparatus, whereas the sound perceptive apparatus from the ectodermal otocyst.
2. The pinna develops from the six hillocks around the 1st branchial cleft.
3. Defective fusion of tubercles gives rise to pre-auricular sinus.
4. The outpouching of the 1st pharyngeal pouch gives rise to a proximal narrow part that forms the eustachian tube and distal dilated part that forms the middle ear cavity.
5. Prussak’s space is a potential space lateral to the sharpnell’s membrane and medially by the neck muscles that can be involved during the extension of cholesteatoma.

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