What makes up the outer ear and where does it run to?
- Pinna: visible structures at each side of the head, responsible of funnelling sound into the ear canal.
- The ear canal, up to the timpanus, is also part of the outer ear structures.
Where is the middle ear?
• Air filled cavity, encased in the temporal bone of the skull.
What are the 3 bones of the middle ear?
What do these bones do?
maellus, incus and stapes
They form a chain physically connecting the timpanus and the oval shaped opening that transmits sound waves into the inner ear.
How are sound waves transmitted to the inner ear?
• The vibration of the timpanus moves the ossicles, and the membrane in the inner ear opening vibrates with ossicle’s movement transmitting the sound waves into the inner ear
What are the two functions of the inner ear?
• This is the compartment morphologically most complex. It has two functions: sound perception and balance.
Where are sound waves in the inner ear perceived?
• Sound perception occurs in the cochlea, where sound waves are transformed into electrochemical stimuli.
How does the ear detect balance?
• Balance is modulated by the dorsal structure in the inner ear, called the vestibule. Three semicircular canals project in the three cardinal axes and are filled with fluid that moves with head movement. This fluid movement is detected at the base of the canals by bundles of sensory cells organised in structures called the crista.
What is the external and middle structure of the ear derived from?
The external and middle structures of the ear are assembled from ectodermal, mesodermal and neural crest derivatives from the first and second pharyngeal arches -
What are the ossicles derived from?
The ossicles derive from first and second pharyngeal neural crest cells, which undergo endochondral ossification.
What is the inner ear and associated neurones derived from?
The inner ear and all the associated neurons derive from the otic placode.
What is the inner ear divided into?
• The inner ear is divided into the dorsal segment and the ventral segment
What is the dorsal segment of the inner ear called and do?
What is at the apical surface of the hair cells found in the vestibule?
- The dorsal segment is called the vestibule is involved in balance, detecting movement and acceleration. It is formed by three canals which contains fluid, this moves when we move and change speed. This movement of fluid is detected by sensory organs that are located at the base of these semi-circular canals. The sensory neurones consist of groups of specialised cells that are called hair cells.
- At the apical surface of hair cells we can see a bundle of stereocilia projecting. This cilia is coupled to each other and are very rigid structure. They become deflected by the movement of the fluid of the semi-circular canals. This movement leads to ion influx within the hair cells which triggers neurotransmitter release and nerve impulses
What does the ventral segment of the inner ear consist of?
What is the organ of corti?
• The ventral segment of the inner ear is the cochlea that is involved in detecting sound. There is an important structure in the cochlea called the organ of corti that runs along the cochlea. The organ of corti is located on one of the walls within the spiral of the cochlea and runs along it, which is formed by hair cells that are able to detect sound
What is the organ of corti?
The Organ of Corti is an organ of the inner ear located within the cochlea which contributes to audition.
What are the 2 cell types in the organ of corti?
Organised into two regions: the inner hair cells row is surrounded by supporting cells and the outer hair cells which consist of three rows of hair cells (they are very stereotypic supported by other cells).
What do the inner hair cells do?
• Connected to auditory nerve fibres. The inner hair cells are the first cells to detect the sound waves and the outer cells mainly function to amplify these sound waves and a bigger array of sounds.
What frequency of sound do hair cells detect in the ar
Cells that detect the lowest frequencies are the hair cells at the apex of the cochlea.
Why as we get older lose the ability to hear high frequency noises?
• As we get older we lose the ability to hear high sound frequencies because the hair cells located at the base of the cochlea are first encountering sound waves and therefore more exposed to the damaging effect of sound.
Recap: what does the inner ear come from?
What is the otic placode?
- The ear and its structures come from the:
- The otic placode is a thickening of the head ectoderm that flanks the hindbrain between rhombomere 5 and 6.
- This thickening starts to invaginate over itself.
What are the different placodes and what do they do?
There are several placodes in the developing embryo such as the olfactory placode, lens placode, trigeminal, optic and epibrachial placode. These will all form neurones that will detect different sensory inputs.
Where do all the different placodes originate form?
All of these placodes derive from a region in the embryo called the Pan Placodal domain. This is a thin row of ectoderm that surrounds the neural plate (this is different to the neural crest).
What signals inhibit and form the pre-placodal region?
What domain forms the otic and epibrachial placodes?
What forms the olfactory, lens and trigeminal placodes?
- Wnts secreted by the neural plate, and BMPs secreted by the epidermis, suppress PPR fate.
- Cerberus (a BMP/Wnt antagonist) and Fgf, secreted by the mesoderm underlying the PPR domain, induce PPR fate.
The PPR is patterned in the AP axis:
• Otic and epibranchial placodes derive from a Gbx2 positive domain
• Anterior placodes derive from a Otx1 positive domain: olfactory, lens and trigeminal placodes
How do the otic and epibrachial placodes form?
What gene is expressed in the OEPD?
- These placodes appear in the gbx2 positive domain, which is at the level of the hindbrain
- Initially there will be a broad domain called the OEPD, which is specified as future placodal tissue, which will be subdivided into a dorsal and medial domain called the OP and Epi
- One characteristic gene that becomes expressed in the OEPD is Pax2.
- Pax2 becomes restricted to the otic vesicle in particular
Describe the signals that form the otic and epibrachial placode
- Here we have a cross section where we can see the neural tube, on the left we can see the posterior PPR and on the right a segregation has occurred
- The posterior PPR is induced by FGFs from the underlying mesoderm
- Wnt8a and FGFs from the hindbrain promote otic placode formation
- Wnt8a suppresses epibranchial placode formation
- Epibranchial fate is induced by Fgfs and Bmps from the pharyngeal endoderm
A – endodermal tissue of the pharyngeal arches is a source of fgf8 that will induce fgfs in the mesoderm.
B – fgfs in the mesoderm will induce Wnts and Fgfs in the hindbrain as the neural plate folds to give rise to the neural tube. In addition, they promote the formation of the posterior PPR from the non-neural ectoderm
C – Wnts and fgfs from the neural tube will induce otic placode fate in the PPR.
D – Wnt will also repress the epibranchial cell fate, so it cannot occur in the PRR. Epibranchial fate also requires induction by fgs and BMPs.
E – so we have FGFs inducing otic region and FGFs and BMPs inducing epibranchial placodes
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The concepts and language of development54
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Animal Models in Development19
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Methods in Development35
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Signalling Pathways42
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Primary Cilla, signalling and development21
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Genome editing and sophisticated transgenesis59
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Neurulation42
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Boundary Formation41
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Somitogenesis34
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Organoids and stem cells30
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Eye development39
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Ear development35
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Neural Crest Part 126
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Neural crest part 217
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Heart development46
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Heart Development Part 223
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Limb Development54
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Limb Development Part 210
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Cortical Development53
