Hearing - Inner Ear Development
Introduction
The inner ear is derived from a pair of surface sensory placodes (otic placodes) in the head region. These placodes fold inwards forming a depression, then pinch off entirely from the surface forming a fluid-filled sac or vesicle (otic vesicle, otocyst). The vesicle sinks into the head mesenchyme some of which closely surrounds the otocyst forming the otic capsule. The otocyst finally lies close to the early developing hindbrain (rhombencephalon) and the developing vestibulo-cochlear-facial ganglion complex.
Some Recent Findings
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Otic Placode
The embryonic surface sensory placode associated with hearing and balance. This will be lost from the embryo surface to form the otocyst or otic vesicle.
- Stage 13/14 embryo - the otic placode has sunk from the surface ectoderm to form a hollow epithelial ball, the otocyst, which now lies beneath the surface surrounded by mesenchyme (mesoderm).
- The epithelia of this ball varies in thickness and has begun to distort, it will eventually form the inner ear membranous labyrinth.
Sensory Placodes
- Week 4 a series of thickened surface ectodermal patches form in pairs rostro-caudally in the head region.
- Recent research suggests that all sensory placodes may arise from common panplacodal primordium origin around the neural plate, and then differentiate to eventually have different developmental fates.
- Each pair of sensory placodes will later contribute key components of each of our special senses (hearing, vision, smell and taste).
- Otic Placode - one of the first to form and contributes inner ear structures.
- Optic (Lens) Placode - lies on the surface, adjacent to the outpocketing of the nervous system (which will for the retina) and will form the lens.
- Nasal Placode - 2 components (medial and lateral) and will form the nose olefactory epithelium.
- Other species have a number of additional placodes which form other sensory structures (fish, lateral line receptor).
- Note that their initial postion on the developing head is significantly different to their final position in the future sensory system.
Otocyst
- week 3 otic placode forms on surface ectoderm
- otic placode sinks into mesoderm
- forms otocyst (otic vesicle)
- branches form and generate endolymphatic duct and sac
- forms vestibular (dorsal) and cochlear (ventral) regions
- differentiation of otic vesicle to membranous labyrinth
Week 4
Stage 13 embryo (week 5) showing otocyst that will form the inner ear.
A. Ventrolateral view of the whole embryo with 5-mm scale bar. At this stage of development no middle or external ear structures are apparent and will be derived later from pharyngeal arches one and two (labeled). | B. The gray bar through the head indicates the plane of cross-section, which is a cross-section of the head showing the size and position of the otic vesicles. At this stage of development they lie within the head mesenchyme behind pharyngeal arch one and two and in close apposition to the developing hindbrain. Note the close position of the otic vesicle to the rhombomeres, hindbrain folds that represent the initial segmentation of the hindbrain. Also shown are developing cranial ganglia and blood vessel lying adjacent to the otic vesicles. The wall of the otic vesicle at this stage is a simple epithelium. |
Week 8
Stage 22 embryo (week 8) showing the embryo near the end of the embryonic period.
A. Lateral view of the whole embryo with 5 mm scale bar. Note the well developed external ear with simplified adult structure and narrower meatal opening. The grey bar through the head indicates the plane of cross-section for (B) and (C). |
B. Cross-section of the head at the plane of the skull base and oral cavity to the top. The otic capsule is well formed by this stage containing all the membranous labyrinth structures. It is still a cartilaginous structure ventral to the brainstem and lying behind the oral cavity. The tongue occupies the floor of the oral cavity with the unfused palatal shelves lying lateral and the auditory tubes clearly shown on the posterior wall. The external ear is visible on the right hand side of the head with a band of cartilage (dark stain) within the auricle. C. The gray box indicates this region: detail of inner and middle ear development. The middle ear cavity has not yet formed and the ossicles (malleus shown) are embedded in mesenchyme that is being lost. The tensor tympani muscle is differentiating in the adjacent mesenchyme. The inner ear membranous labyrinth has formed its adult external structure. The section through the turns of the cochlear duct shows the internal cochlea structure is still underdeveloped; in contrast, the balance region is more developed. |
Vestibular Sac
- generates 3 expansions - form semicircular ducts
- remainder forms utricle
- epithelia lining generates - hair cells, ampullary cristae, utricular macula
- Vestibular - Otoconia, otoconin- inner ear biominerals
Cochlear Sac
- generates coiled cochlear duct (humans 2 1/2 turns)
- remainder forms saccule
- epithelia lining generates
- hair cells
- structures of organ of corti
- saccular macula
Cochlea
Magnetic Resonance images of the adult Cochlea[2] | |
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The investigators used a method to obtain magnetic resonance imaging (MRI) to measure cochlear length from the temporal bones of 6 cadavers. By overlapping digitalized rulers on these images it was possible to measure cochlear length. Adult cochlear length varied between 17 and 26.5 millimeters.[2]
Bony Labyrinth
- formed from chrondified mesoderm
- Periotic Capsule
- mesenchyme within capsule degenerates to form space filled with perilymph
Auditory Neural Pathway
Vestibulocochlear Nerve
- forms beside otocyst
- from wall of otocyst and neural crest cells
- bipolar neurons
Vestibular Neurons
- outer end of internal acoustic meatus
- innervate hair cells in membranous labyrinth
- axons project to brain stem and synapse in vestibular nucleus
Cochlear Neurons
- cell bodies lie in modiolus
- central pillar of cochlear
- innervate hair cells of spiral organ
- axons project to cochlear nucleus
- Links: Hearing - Neural Pathway
Inner Ear Genes
- hindbrain segmentation occurs at same time placode arises
- otocyst adjacent to rhombomere 5
- may influence development
- Hoxa1, kreisler, Fgf3
- genes regulating neural crest cells (neural genes)
- Pax2 Ko affects cochlear and spiral ganglion, but not vestibular apparatus
- nerogenin 1 affects both ganglia
Semicircular canal
- Otx1- cochlear and vestibular normal
- Hmx3, Prx1, Prx2
Sensory Organs
- thyroid hormone receptor beta
- Zebrafish-mindbomb mutant has excess hair cells but not supporting cells, Notch-Delta signaling
- Gene Expression-inner ear
- Brn-3c and Hair cell development
- Supporting Cells- p27kip
- Thyroid Hormone
- Ganglion neurons require growth factors
- vestibular neurons- BDNF, NT3
- survival not development
Sox9 20346939 Sox2 20071536
References
- ↑ <pubmed>19476657</pubmed>
- ↑ 2.0 2.1 <pubmed>19575114</pubmed>| Braz J Otorhinolaryngol.
Reviews
<pubmed>20637105</pubmed> <pubmed>19750520</pubmed>
Articles
<pubmed>18603386</pubmed>| PMC2628570
Search PubMed
May 2010 "Inner Ear Development" All (4027) Review (452) Free Full Text (750)
Search Pubmed: Inner Ear Development Cochlea Development
External Links
- Neuroscience Neuroscience - The Inner Ear
Glossary Links
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Cite this page: Hill, M.A. (2024, June 15) Embryology Hearing - Inner Ear Development. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Hearing_-_Inner_Ear_Development
- © Dr Mark Hill 2024, UNSW Embryology ISBN: 978 0 7334 2609 4 - UNSW CRICOS Provider Code No. 00098G