Development of Ear

00:01 Hello. We are now gonna discuss development of the ear.

00:04 Both the inner, middle, and outer portions.

00:07 To start with, we have a thickening of the ectoderm overlying the rhombencephalon.

00:15 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.

00:23 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.

00:33 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.

00:43 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.

00:58 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.

01:12 So once it´s within the mesenchyme, it´s going to develop an extension that goes dorsally and then, an extension that goes ventrally.

01:21 So these dorsal and ventral compartments will form different portions of the inner ear.

01:26 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.

01:37 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.

01:45 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.

01:59 So we´ve got a lot of things meeting in this exact area as the inner ear´s developing.

02:05 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.

02:17 We´re gonna move a little further along and we can see a very clear dorsal and ventral compartment.

02:22 The endolymphatic sac is stretching superiorly and now, we can see that the stapes is getting close to the inner ear.

02:31 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.

02:43 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.

02:55 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.

03:05 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.

03:19 Keep in mind that we still have the endolymphatic duct and sac extending off of it.

03:23 Whereas the ventral side, it´s gonna create the saccule and the cochlea.

03:28 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.

03:40 Now, moving further along towards almost completely mature development.

03:44 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.

03:57 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.

04:20 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.

04:30 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.

04:39 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.

04:50 And that surrounding tissue is gonna develop little gaps in it, little vacuoles that are going to expand, fuse, and create a coherent space.

04:59 As it does so, it´s gonna fill with a different fluid called perilymph.

05:03 Perilymph surrounds the organs of the ear and the organs of the ear within the derivatives of the otic vesicle are full of endolymph.

05:13 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.

05:25 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.

05:34 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.

05:47 So air vibrations hit our eardrum.

05:50 Our eardrum moves our ossicles, the malleus, the incus, the stapes.

05:54 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.

06:10 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.

06:24 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.

06:39 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.

06:49 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.

07:08 But in the case of the semicircular canals, they depolarize in response to changes in acceleration.

07:13 And the saccule and utricle depolarized in response to changes in position of our head relative to gravity.

07:20 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.

07:42 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.

07:57 But endolymph is distinct from cerebrospinal fluid.

08:00 As opposed to that, the perilymph is in direct contact with the cerebrospinal fluid through what´s called the perilymphatic duct.

08:09 So perilymph and cerebrospinal fluid are in direct contact whereas endolymph is chemically distinct from them.

08:16 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.

08:29 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.

08:50 Now, there are six auricular helix that develop on these arches.

08:54 One, two, three on the first arch and four, five, six on the second arch.

08:59 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.

09:10 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.

09:21 So you may need to do a more thorough check on a newborn with any sort of external ear malformation.

09:27 Okay, thank you very much and I´ll see you for our next talk.