Hello. We are gonna discuss the pharyngeal grooves and pharyngeal pouches.
Now, this topic gave me a terrible time when I was trying to learn it
so I´ve done my best to adopt a beginner´s perspective on it
and try to address it in the most straightforward way possible.
So let´s remember a little bit ago, we discussed how the tongue forms
and that we have the stomodeum or early mouth
and inside it, we have endoderm lining the various pharyngeal arches.
So in the process of lining those arches, there are little separations
between the first arch and the second arch,
the second arch and the third arch and so forth.
On the inside, those are gonna be called pharyngeal pouches
and on the outside, they´re gonna be called pharyngeal grooves or pharyngeal clefts.
Now, since they´re on the outside,
the pharyngeal grooves are lined by ectoderm
and since the pharyngeal pouches are on the inside,
they´re gonna be lined by endoderm.
And because of that, they come up with very distinct adult structures
that develop off of them.
Now, the first pharyngeal groove is on the outside.
Since it´s the first groove,
it´s located between the first arch and the second arch.
This first pharyngeal groove grows deeper and it grows into the developing neck
and approaches the first pharyngeal pouch.
So these two are growing together to meet somewhere in the neck.
Now, the second, third, and fourth pharyngeal grooves disappear.
What typically happens is that the second pharyngeal arch grows extensively
and actually grows over the third and fourth arch and fuses with the sixth arch.
This leaves a small little space present
where there used to be the second, third, and fourth pharyngeal grooves.
This space is called the cervical sinus
and if things go normally, it´s going to disappear and leave nothing behind
and those grooves don´t really contribute anything to the mature person.
However, if they don´t disappear,
what´s gonna happens is you can have a cervical cyst
and that cervical cyst is a persistent remnant of that embryologic structure.
Typically, they´re fairly innocent but if they get aggravated,
they can swell and become painful.
And people don´t tend to like it
when they have something swelling and enlarged in their neck.
In the case of cervical cysts they´re almost
always located anterior to the sternocleidomastoid muscle
and they can be connected to the outside by an external fistula
or to the inside of the pharynx by an internal fistula.
Now, if you have an external fistula,
that means you´re gonna have a little hole in the front of your neck,
right in front of the sternocleidomastoid muscle
and it´s gonna occasionally drain mucus and some other nasty stuff.
If you have an internal fistula, people generally aren´t aware of that
but there is one very interesting problem they complain of, bad breath.
Because you´ve got this opening in your neck with a possible cyst on the other end,
it can catch bits of food that aren´t swallowed
and then hang out on that cyst and go bad.
So persistent bad breath is sometimes associated with these internal cervical fistulas
and to complete the picture that we´ve painted here,
you can occasionally have an internal fistula
that connects to a cyst that connects to an external fistula.
So you have an unbroken hole through the side of your neck
to the inside of your pharynx.
These are fairly uncommon but they are known to occur.
Now, we´ve discussed the grooves.
Let´s move inside and talk about the fates of the pharyngeal pouches.
The first pouch grows outward.
Now, remember, the first pouch is on the inside
and it´s located between the first pharyngeal arch and the second pharyngeal arch.
It´s going to extend towards the first groove
and it´s actually gonna develop into the middle ear and the auditory tube.
The connection between the middle ear and the pharynx.
The second pharyngeal pouch only deepens a bit.
It doesn´t quite go as extensively as the first pouch, it just deepens a tiny bit
and the third and fourth pharyngeal pouches do an interesting thing.
They grow outward but they develop a ventral and dorsal little division.
So it actually splits in two as it deepens.
As development proceeds, the first groove
and the first pouch come very close together
until only a thin membrane separates them.
And the membrane that separates the external ear from the middle ear
is in fact the tympanic membrane
and that´s the point where you give way
between the first groove on the outside and the first pouch on the inside.
The second pharyngeal pouch deepened a bit
but it just serves as the bed for the tonsil.
So your palatine tonsil has lymphatic tissue migrate in
and hang out in the little divot that the second pharyngeal pouch created.
So the pouch does not make the tonsil,
it just serves as the bed for those lymphatic cells that migrate in.
Now, the third and fourth pharyngeal pouches
are where things really get interesting.
Those little divisions, the ventral and dorsal ones deepen
and extend even further into the underlying mesenchyme
and they´re gonna become glandular structures.
The third pharyngeal pouch
is gonna create the inferior parathyroid gland and the thymus.
Now the parathyroid glands regulate our blood calcium
and when they signal parathyroid hormone into the blood stream,
they´re going to try to increase the amount of blood calcium.
The thymus gland is involved with maturation of T-cells and our immune response.
Now, the fourth pharyngeal pouch also produces a parathyroid gland.
In this case, the superior parathyroid gland
and something called the ultimopharyngeal body.
Also sometimes called the ultimobranchial body.
So if you see that term, they really just mean the same thing.
This structure is going to give rise to C-cells that are located in the thyroid gland.
These C-cells produce calcitonin
and that´s another calcium regulating hormone
and increased levels of calcitonin are going to try to lower the serum calcium.
So between the third and fourth pharyngeal pouches,
you´ve got the mechanism in place to increase
or decrease the amount of calcium in your body.
Now, one question I get asked a lot
is why on earth does the inferior parathyroid gland come from the third pouch
but the superior one come from the fourth pouch which is more inferior.
I don´t have a very satisfying explanation for that
other than as the thyroid gland migrates,
it´s going to encounter that third pharyngeal pouch first,
pick up the parathyroid gland and carry it with it,
then, as it goes a little further,
it then picks up the superior parathyroid gland
and continues on down to its mature position in the trachea.
With the parathyroid glands stuck into its posterior substance
of the lateral lobes of the thyroid gland.
Now, what can go wrong?
Occasionally, you can have one or more pharyngeal pouch structures fail to form
and you just don´t have the same number of parathyroid glands
or one half of your thymus present.
If the parathyroid glands don´t reach their mature position,
they can still be functional
but you wanna watch out to that if you have a parathyroid tumor,
you may have more than one parathyroid gland releasing extraneous
or extra amounts of its hormone.
So these undescended or ectopic parathyroid glands are clinically not too important
unless they go bad and you have to track down where they´re located
if they´re not in their normal position in the thyroid.
The thymus may fail to descend fully
or sometimes leave a little strip of thymic tissue stretching
back up the neck towards its origin in the third pharyngeal pouch.
Now, one very commonly described syndrome
involving the thymus gland is DiGeorge syndrome.
DiGeorge syndrome is due to a partial deletion of chromosome 22
and there are several problems that are present with this syndrome,
heart defects, cleft palate, language delays, and learning difficulties
are all associated with DiGeorge syndrome
but in particular, we have hypocalcemia and frequent infections.
Those seem like unconnected complaints
but they´re connected by the fact that the third pouch,
the third pharyngeal pouch creates the parathyroid glands and the thymus
and if you have a problem with those, you can have hypocalcemia
because you don´t have enough calcium being mobilized by parathyroid hormone.
And if you don´t have a place for your T-cells to mature
which is typically what your thymus gland is doing,
you can wind up with a more than regular amount of infections coming at you
because your T-cells are not doing a good job of fighting off infection
and this is all based around their origin in the pharyngeal pouches.
Thank you very much and I´ll see you in the next talk.