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 11 - single layer of ectodermal cells organized in a columnar epithelium, which differs in cell shape from the surrounding cuboidal epithelia that will contribute the epithelia of the skin.
- zebrafish model - a number of specific genes are involved in initial induction of the otic placode including both growth factors (fgf3 and fgf8) and transcription factors (dlx3b, dlx4b, and foxi1).[2]
- Proliferation of the otic placode cells leads to an inward folding, or invagination, giving the external appearance of a depression on the lateral sides of the early developing neck region.
- The epithelium is still a single layer of cells, which continues to invaginate until the edges of the disc of cells come into apposition on the embryo surface.
- mouse model - placodal invagination but not specification requires placodal expression of the transcription factor Sox9.[3]
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
Stage 13 otic vesicle now lies beneath the embryo surface.
- Stage 13 - the otic placode has sunk from the surface ectoderm to form a hollow epithelial ball, the otocyst (otic vesicle), which now lies beneath the surface surrounded by mesenchyme (mesoderm and neural crest).
- The epithelia of this ball varies in thickness and has begun to distort, it will eventually form the inner ear membranous labyrinth.
- forms otocyst
- branches form and generate endolymphatic duct and sac
- forms vestibular (dorsal) and cochlear (ventral) regions
- differentiation of otic vesicle to membranous labyrinth
Week 5
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 7
Week 8
Stage 22 embryo (week 8) showing the embryo near the end of the embryonic period.
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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. |
Fetal
A series of historic wax-plate reconstructions of the membranous labyrinth and the surrounding periotic tissue-spaces showing development stages (median and lateral views) of these spaces at the same scale (ages are only approximations calculated on CRL).
lateral view |
median view |
Fetus 50 mm CRL
10 weeks (GA12) |
lateral view |
median view |
Fetus 85 mm CRL
14 weeks (GA15) |
lateral view |
median view |
Fetus 130 mm CRL
15 weeks (GA17) |
Endolymphatic Sac
- the adult endolymphatic sac is filled with endolymphatic fluid
- with a unique composition of high potassium and low sodium ions
- it is not known in humans at what stage of development this ionic status is achieved
- In the rat, adult sodium levels are seen in the first week after birth, while both potassium and chloride levels were below the normal adult levels.
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
scala media
(Latin, medius = middle) spiral of middle cochlear duct lying between scala vestibuli and scala tympani, containing endolymph.
scala tympani
(Latin, tympanon = drum) the spiralling cochlear duct below spiral lamina, containing perilymph and ending at round window near tympanic membrane.
scala vestibuli
(Latin, vestibulum = cavity at beginning of canal) the spiralling cochlear duct above spiral lamina, containing perilymph, beginning near the vestbule and ending where it communicates with the scala tympani at the helicotrema.
endolymph
- extracellular fluid secreted by the stria vascular is.
- potassium is the main cation required for depolarizing electrical current in the hair cells.
perilymph
- extracellular fluid similar in composition to either plasma or cerebrospinal fluid.
- sodium is the main cation.
Cochlea
Fetus 50 mm CRL (lateral)
Fetus 50 mm CRL (median)
Fetus 85 mm CRL (lateral)
Fetus 85 mm CRL (median)
Fetus 130 mm CRL (lateral)
Fetus 130 mm CRL (median)
Cochlear and vestibular specification
Mouse - inner ear cartoon
| Magnetic Resonance images of the adult Cochlea[4] | |
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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.[4]
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
Historic Images
Kollmann Fig. 731
Kollmann Fig. 732
Kollmann Fig. 733
Kollmann Fig. 734
Kollmann Fig. 735
Kollmann Fig. 736-737
Kollmann Fig. 738
Kollmann Fig. 739
Kollmann Fig. 740
Kollmann Fig. 741
Kollmann Fig. 742
Kollmann Fig. 743
Kollmann Fig. 744
Kollmann Fig. 745
Kollmann Fig. 746
Kollmann Fig. 747
Kollmann Fig. 748
Lewis Fig. 16 Dorsal view of temporal and occipital cartilages, showing the relation of the inner ear to the otic capsule.
References
Reviews
<pubmed>20637105</pubmed> <pubmed>19750520</pubmed>
- Bookshelf - Neuroscience Neuroscience - The Inner Ear
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
External Links Notice - The dynamic nature of the internet may mean that some of these listed links may no longer function. If the link no longer works search the web with the link text or name. Links to any external commercial sites are provided for information purposes only and should never be considered an endorsement. UNSW Embryology is provided as an educational resource with no clinical information or commercial affiliation.
- Mouse Genomic Database cochlea morphogenesis genes
- Acoustical Society of America ICA/ASA '98 Lay Language Papers
- Promenade around the Cochlea Organ of Corti
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Cite this page: Hill, M.A. (2024, May 1) 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
