- There are three morphological divisions of the ear. They are: the external, middle and internal ear.
- These all divisions have a separate origin.
- The formation of membranous labyrinth occurs from a specialized area of surface ectoderm overlying the developing hindbrain.
- At first, this area is apparent as a thickening called the otic placode.
- As the otic placode becomes depressed there is formation of the otic pit.
- This otic pit becomes rounded and separates from the surface ectoderm to form otic vesicle which is an oval structure at first.
- There is differential growth of various parts of its wall by which it gives rise to structures comprising the membranous labyrinth.
- Specialized sensory end organs of hearing and of equilibrium (cristae of semicircular ducts maculae of utricle and saccule, organ of corti of cochlea) are formed by the differentiation of localized areas of the epithelium of the membranous labyrinth.
- Peripheral processes of the cells of the vestibulo-cochlear ganglion innervate these structures later.
- This ganglion is derived from the neural crest which cells are unique as they remain bipolar throughout life.
- Mesenchyme gives rise to bony labyrinth that surrounds the membranous labyrinth.
- This mesenchyme later on gets condensed to form the otic capsule, which soon gets converted into cartilage.
- There exists a layer of loose periotic tissue between the cartilage and the membranous labyrinth.
- This periotic tissue disappears after which the spaces of bony labyrinth are created.
- The fluid called endolymph fills the membranous labyrinth while the periotic spaces surrounding it are filled with perilymph.
- A space called the vestibule is formed after the disappearance of periotic tissues around the utricle and saccule.
- Formation of semicircular canals also occurs after periotic tissues around the semicircular ducts disappear.
- On either side of cochlear duct two distinct spaces are also formed called scala tympani and scala vestibuli.
- There is communication of scala vestibuli with vestibule while the scala tympani grow towards the tympanic cavity from which it remains separated by a membrane.
- The cartilaginous labyrinth is subsequently ossified to form the bony labyrinth.
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- Tubo-tympanic recess is the structure from where the epithelial lining of the middle ear and pharyngo-tympanic tube occur.
- This recess develops from the dorsal part of the first pharyngeal pouch and also receives a contribution from the second pouch.
- The extensions from the middle give rise to the tympanic antrum and mastoid air cells.
- The dorsal end of Meckel’s cartilage gives rise to malleus and incus while the dorsal end of cartilage of the second pharyngeal arch gives rise to stapes.
- Outside the mucous membrane of the developing middle ear, lie the ossicles, which invaginate the mucous membrane covering them throughout life.
- By the fourth month of the intrauterine life, the ossicles of the ear fully ossify being the first bones in the body to do so.
- Likewise, the mesoderm of the first pharyngeal arch gives rise to tensor tympani and the second arch to stapedius.
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- The dorsal part of the first ectodermal cleft gives rise to the external acoustic meatus.
- However, the proliferation of its lining epithelium forms its deeper part which grows towards the middle ear.
- This proliferation gets canalized later which is initially a solid (meatal plug).
- Six mesodermal thickenings called tubercles or hillocks give rise to the auricle or pinna that appears on the mandibular and hyoid arches.
- These are located around the opening of the dorsal part of the first ectodermal cleft (around the opening of the external acoustic meatus).
- The tragus is formed only by the mandibular arch whereas a small area around it gives rise to the rest of the auricle being formed from the hyoid arch.
- This is consistent with the fact that the auricular muscles are supplied by the facial nerves.
- The tubo-tympanic recess and the first ectodermal cleft apposition lead to the formation of tympanic membrane.
- These two structure forms the inner endodermal and outer ectodermal epithelial linings of the membrane.
- The intervening mesoderm forms the connective tissue basis.
Development of the Ear