Hello. We are now gonna discuss development of the ear.
Both the inner, middle, and outer portions.
To start with, we have a thickening of the ectoderm overlying the rhombencephalon.
So if you can think back to how the ear´s connected to the brain,
the vestibular and cochlear nuclei tend to be in the medulla.
So what´s going to happen is that the overlying ectoderm
is going to thicken to form an otic placode
which is then going to deepen into a pit.
We´ve seen the same process happen in both the nasal and eye development
and here, when the ear is developing,
the otic pit is growing inward towards the rhombencephalon.
As it´s doing so, it´s going to associate with some neural crest derived cells
that are gonna be forming the statoacoustic ganglion
that´s going to associate with derivatives of the inner ear
and hence, cranial nerve number eight.
Now, the otic pit moves into the underlying mesenchyme
and actually detaches from the surface entirely forming an otic vesicle
and this otic vesicle is gonna be what forms
pretty much the entirety of the inner ear.
So once it´s within the mesenchyme, it´s going to develop an extension that goes dorsally
and then, an extension that goes ventrally.
So these dorsal and ventral compartments will form different portions of the inner ear.
Note that it´s very close to some ossicle condensations
and these condensations will form the three bones of the middle ear,
the malleus, the incus, and the stapes.
Now, these are coming from the first and second pharyngeal arches
and they´re not the only thing that´s going on in this area.
We also have an extension of the first pharyngeal pouch
coming upward to meet in extension of the first pharyngeal groove
and those are gonna form the auditory canal and middle ear canal,
as well as the external ear.
So we´ve got a lot of things meeting in this exact area as the inner ear´s developing.
Now, the dorsal compartment has an extension called the endolymphatic appendage
that´s going to reach towards the developing space of the brain
and underlie the dura mater once it comes into existence.
We´re gonna move a little further along
and we can see a very clear dorsal and ventral compartment.
The endolymphatic sac is stretching superiorly
and now, we can see that the stapes is getting close to the inner ear.
That makes sense because the malleus is gonna conduct vibrations
from the eardrum to the incus, to the stapes
which actually moves the fluid within the inner ear.
Now, as this is happening, the external ear canal
is deepening the external acoustic meatus
and it's moving closer and closer to that extension
of the first pharyngeal pouch, the auditory tube.
Moving a little further along, we can see that the auditory tube is enlarging
and it´s actually going to start surrounding the ossicles to create the tympanic cavity.
But in the meantime, we´ve got the dorsal
and ventral extensions of the otic vesicle really becoming distinct
and the dorsal portion is gonna create the semicircular canals as well as the utricle.
Keep in mind that we still have the endolymphatic duct and sac extending off of it.
Whereas the ventral side, it´s gonna create the saccule and the cochlea.
Now, if you know your anatomy, you´ll know that the cochlea is a spiral organ
but at this point, it´s just a little horn sticking down but it will eventually elongate
and form that characteristic spiral of the cochlea.
Now, moving further along towards almost completely mature development.
We can see that the semi-circular canals and saccule are present superiorly
and we´ve got the saccule connecting to the cochlea
just a bit inferiorly also in the ventral compartment.
The stapes is in close contact with the inner ear
and the tympanic cavity of the middle ear is in completely around those ossicles
and it´s connected to the auditory tube
which in turn, connects the middle ear to your pharynx
and this is one reason that middle ear infections can be tied to throat infections
because there´s a complete open canal between the two spaces.
The tympanic cavity has also come into close association
with the external acoustic meatus
which is creating the tympanic membrane or eardrum at their interface.
Now, let´s take a closer look at what´s happening as the otic vesicle
and its derivatives form the actual structures of the inner ear.
We´re gonna work on the cochlea first and see that the cochlear duct
which is full of endolymph derived from the otic vesicle
is surrounded by some mesenchyme and cartilage.
And that surrounding tissue is gonna develop little gaps in it,
little vacuoles that are going to expand, fuse, and create a coherent space.
As it does so, it´s gonna fill with a different fluid called perilymph.
Perilymph surrounds the organs of the ear
and the organs of the ear within the derivatives of the otic vesicle are full of endolymph.
So endo means inside, so endolymph is indeed inside the inner ear sensory organs
and the perilymph, peri means surrounding or around
is gonna literally surround them.
And as those spaces open up, we can see in the cochlea
that the perilymph spaces are gonna become the scala tympani, the scala vestibuli.
So a space that´s full of perilymph surround the derivatives of the otic vesicle
which is full of endolymph
and this is what allows sound to actually be translated
from vibrations in the air to nerve conduction.
So air vibrations hit our eardrum.
Our eardrum moves our ossicles, the malleus, the incus, the stapes.
The stapes hits the oval window
and creates a fluid motion inside the endolymph
and as that endolymph has waves pass through it,
it´s going to deflect hair cells that develop within the organ of corti.
Now, the organ of corti is part of that cochlear duct derived from the otic vesicle
and it´s associated with cranial nerve eight,
specifically, the spiral ganglion sends nerves into the organ of corti.
And as those hair cells are deflected as the fluid moves,
they´re gonna create depolarization of those nerves
and that´s what translates vibration of the air into actual nervous impulses
that we are understanding as hearing.
A similar thing happens in the semicircular canals, the saccule, and the utricle
whereas the hairs deflect in response not to sound,
but to gravity and acceleration.
So cranial nerve eight is gonna be derived from that statoacoustic ganglion
that´s gonna send cells not only into the cochlea
but into the saccule, the utricle, and the semicircular canals,
and a similar process occurs there
where deflection of sensory hair cells is gonna cause depolarization of those nerves.
But in the case of the semicircular canals,
they depolarize in response to changes in acceleration.
And the saccule and utricle depolarized in response to changes
in position of our head relative to gravity.
The semicircular canals take on their unique shape
because the spaces in between them are going to pinch close
and give us three sets of tubes in the X plane, the Y plane, and the Z plane,
but they remain full of endolymph and are connected to the utricle,
the saccule, and the endolymphatic duct.
Now, the endolymphatic duct is going to stretch upwards
to a spot just underneath the dura where the endolymphatic sac
allows some recycling of the ions
and other chemicals in the endolymph into the CSF.
But endolymph is distinct from cerebrospinal fluid.
As opposed to that, the perilymph is in direct contact
with the cerebrospinal fluid through what´s called the perilymphatic duct.
So perilymph and cerebrospinal fluid are in direct contact
whereas endolymph is chemically distinct from them.
Now, the middle ear comes into existence
because the first pharyngeal pouch reaches up,
surrounds the ossicles and creates the tympanic cavity
and it associates with the external acoustic meatus to form the ear drum.
The external acoustic meatus comes from the groove
between the first pharyngeal arch on the outside
and the second pharyngeal arch on the outside
and that space between them is what´s gonna form the external ear canal
and because of that, the first and second pharyngeal arches
form the external ear or oracle.
Now, there are six auricular helix that develop on these arches.
One, two, three on the first arch and four, five, six on the second arch.
They´re gonna migrate around each other to create all the structures of the external ear
and they sometimes have little faults in migration
and have an unusual appearance or little ear tags.
Those are not problematic in and of themselves
but they often are signs that there may be first
and second pharyngeal arch derivatives that have other problems in them.
So you may need to do a more thorough check on a newborn
with any sort of external ear malformation.
Okay, thank you very much and I´ll see you for our next talk.