Hello. And now, we´re gonna discuss the pharyngeal arches.
Now, these go by a variety of names.
Pharyngeal arches, gill arches, branchial arches,
but they´re all referring to the same thing which is a series of mesenchymal folds
that develop along the neck during early, early development.
Before we have a face, we start to get these pharyngeal arches.
Now, they´re basically a little curving arch with the hanging loose side
pointing inferiorly that´s gonna be full of mesenchyme.
And that mesenchyme is gonna develop into a variety of different structures.
As development proceeds from anterior to posterior, we develop five sets of arches.
So the first arch will be located anterior
to the second, to the third, to the fourth, and so on.
Now, these arches are gonna form a variety of structures
including most of our lower face.
They´re gonna be part of our ears, our throat,
and along with that, several glands that contribute to our endocrine system
and especially, to our calcium regulation.
So each pharyngeal arch contains a core that´s gonna have dense mesenchyme
within it that´s going to become a cartilage rod which will then later, transition into bone,
so will undergo endochondral ossification
and that core of cartilage is associated with a muscle,
and that muscle is associated with a nerve,
and there´s also gonna be an artery supplying it.
Just a quick preview of heart development will tell you
that the artery within each pharyngeal arch is called an aortic arch
and they´re present here
and they´re also gonna be contributing to most of our vasculature in the neck and head.
So that´s a brief preview of coming attractions.
Now, the first pharyngeal arch, the one that´s closest to the front
is gonna become part of the incus and malleus inside of our ear.
So incus and malleus, the first two ossicles
as well as our mandible, our jaw, and parts of the maxilla.
So the zygomatic bone, maxilla, are gonna be coming from the first arch.
Going a little further inferiorly, we have the second arch.
It´s going to form the last of the three ossicles
so the middle ear bones, the stapes, the styloid process of the skull
and part of the hyoid bone, specifically, its lesser horn
are all coming from the second arch.
The third arch is gonna create the body
and greater horn of the hyoid bone.
And the fourth and sixth arches create laryngeal cartilages.
Now, if that´s a bit abstract for you, I´ve got some good news.
This picture should make it clear that we´re laying down these arches
from superior to inferior with the first arch
contributing the malleus, the incus, parts of the mandible.
The second arch, the stapes, the styloid process,
and the lesser horn of the hyoid bone.
The rest of the hyoid bone is coming from the third arch.
And finally, the fourth and sixth arches do not ossify
but they remain cartilaginous and create the laryngeal cartilages.
Now, if you look to the left, you´ll see that a variety of cranial nerves
are also growing into the pharyngeal arches.
In this case, cranial nerves 5, 7, 9, and 10.
And right now, it´s a good idea just to drill those numbers 5, 7, 9, 10,
because they´re gonna be very important as this development proceeds.
Now, in the head and neck, we have structures called somitomeres
and they´re very much like somites that we´ve seen earlier
and remember that somites were distinct little beads
developing on either side of the neural tube that were innervated by a nerve
and then, drag that nerve behind it as it would migrate
to become different muscles in the body.
Somitomeres are very similar
except they don´t form distinct bulges like the somites
and we´re gonna have somitomeres migrate into each arch
and as they do so, they´re gonna drag their nerve supply behind them.
That nerve supply is coming from cranial nerves 5, 7, 9, and 10.
So as they move into the arch,
they´re going to associate with the various cartilaginous
and bony structures that are developing there.
And that cranial nerve will thereafter not just innervate those muscles
but move whatever cartilaginous and bony structures those muscles are attached to.
Now, the first arch, the somitomeres that migrate into it
are going to form the muscles of mastication.
AKA the muscles that we use to eat.
Now, by necessity, those muscles have to attach to the jaw.
So muscles of mastication as well as part of our digastric muscle
and our tensor tympani muscle all come from the first arch.
Now, the first arch gave rise to the incus, the malleus, the mandible,
as well as some of the facial bones.
So it makes sense that the muscles that are going to be attaching to those bones
are coming from the first arch
and the main thing to remember about their innervation
is that they´re all innervated by V3, the mandibular branch of the trigeminal nerve
and anytime that nerve has to be dragged behind those muscles as it moves,
it´s going to leave a course through the bone.
Next up, we have the second pharyngeal arch.
The somitomeres that migrate in here are pulling behind them
the branches of the facial nerve.
So the facial nerve innervates all of these somitomeres
and they then, migrate to any bones that are gonna be associated
with the second arch.
So the styloid process, part of the hyoid bone and the stapes.
That tiny little stapedius muscle inside the middle ear is innervated by the facial nerve
because the stapes comes from the second arch.
Now, much more prominently, the muscles of facial expression
that migrate from their starting point about here all over the face
are gonna pull the facial nerve behind them
and that´s why the facial nerve has that extended branching pattern on the face
as it moves around and is dragged as the muscles migrate.
So as the muscles migrate across the face,
the facial nerve is passively pulled behind.
Now, an interesting exception to this rule is the third arch.
It´s not an exception in that it does anything that different but it creates very little muscle.
In fact, the somitomeres that go into the third arch
eventually create a single muscle called the stylopharyngeus
and that muscle is a longitudinal muscle inside the pharynx.
It´s innervated by the glossopharyngeal nerve
and the glossopharyngeal nerve innervates
one and only one somitomere-derived muscle and that´s it.
Now, as opposed to the third arch, the fourth arch gives rise to a great many muscles.
All the muscles of the palate and the pharynx except for the stylopharyngeus
are all derived from the fourth arch
and the fourth arch is innervated by the vagus nerve.
So when you associate motor activity of the vagus nerve with anything,
it´s probably the palate, the pharynx, and the larynx.
Now, let´s move on to the sixth pharyngeal arch.
The somitomeres that enter the sixth arch are also innervated by the vagus nerve
but they´re specifically innervated by its recurrent branch.
A very separate and distinct branch of the vagus nerve
goes into the sixth arch, innervates the muscles that are found there
and these muscles will become all the muscles of the larynx,
the ones that allow us to speak.
So all of our muscles of phonation are coming from the sixth arch
and are innervated by the recurrent branch of the laryngeal nerve
or the current laryngeal nerve which is a branch of the vagus nerve.
The only exception is the one from the fourth arch, the cricothyroid muscle.
Now, you may be thinking to yourself that I made an error
and I didn´t talk about the fifth arch.
The fact is these structures were described first in fishes and gilled animals
and there were in fact one, two, three four, five, six,
sometimes seven gilled structures inside these fishes.
When it came time to organize human embryology
and figure out what was going on, they found that the fifth arch structures
that exist in some animals have no homologs in humans.
That´s why humans have one, two, three, four, no five, and a sixth pharyngeal arch.
Now, let´s talk about things that can go wrong with the first arch.
There are a variety of problems.
Treacher-Collins syndrome, Pierre-Robin sequence, and Agnathia.
Now, Treacher-Collins syndrome results from too few neural crest cells
migrating into the first arch
and too little tissue there to actually build things.
So we have underdevelopment of the jaw.
That´s called micrognathia.
You´re gonna have deformity of the zygomatic bones, part of the bony structures
that come from the first arch in the cheek as well as the ossicles and external ears.
So difficulty hearing is another hallmark of Treacher-Collins syndrome.
And mutations are resulting in too few neural crest cells
migrating here in the first place.
So it´s a genetic issue that´s brought about with too few neural crest cells
making it to the first arch in the first place.
Something that may initially appear similar but is very distinct is Pierre-Robin sequence
and a sequence implies one thing goes wrong and sets off a chain of effect thereafter.
In the case of Pierre-Robin sequence, you´ve got a small jaw.
And that small jaw can occur for reasons that we don´t quite understand.
Most prevalent theory right now is that its intrauterine positioning
and the jaw is pressed against the uterus or against some other structure
and is unable to develop.
And because the jaw can´t develop anteriorly,
we wind up with too small a jaw, micrognathia
but it also means that the tongue is too far posterior
and that does not allow the palate to close, so we have a cleft palate.
So the Pierre-Robin sequence is micrognathia, posterior tongue, and cleft palate.
Last and quite possibly less common than anything else, we have agnathia.
Failure of the jaw to form in the first place.
This is a severe problem related to migration of neural crest cells into the first arch
and flat out failure of the first arch to form normally.
Thankfully, this is a very uncommon thing to encounter
but it does have a name and it does occur to some degree.
So be on the watch for it if you´re ever on pediatric or OB-GYN rotations
and looking for something very strange on ultrasound.
Alright, thank you very much.