Pharyngeal Grooves and Pouches

00:00 Hello. We are gonna discuss the pharyngeal grooves and pharyngeal pouches.

00:06 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.

00:16 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.

00:30 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.

00:40 On the inside, those are gonna be called pharyngeal pouches and on the outside, they´re gonna be called pharyngeal grooves or pharyngeal clefts.

00:49 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.

00:59 And because of that, they come up with very distinct adult structures that develop off of them.

01:06 Now, the first pharyngeal groove is on the outside.

01:10 Since it´s the first groove, it´s located between the first arch and the second arch.

01:15 This first pharyngeal groove grows deeper and it grows into the developing neck and approaches the first pharyngeal pouch.

01:24 So these two are growing together to meet somewhere in the neck.

01:28 Now, the second, third, and fourth pharyngeal grooves disappear.

01:34 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.

01:45 This leaves a small little space present where there used to be the second, third, and fourth pharyngeal grooves.

01:54 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.

02:05 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.

02:15 Typically, they´re fairly innocent but if they get aggravated, they can swell and become painful.

02:22 And people don´t tend to like it when they have something swelling and enlarged in their neck.

02:27 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.

02:41 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.

02:52 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.

03:01 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.

03:13 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.

03:27 So you have an unbroken hole through the side of your neck to the inside of your pharynx.

03:32 These are fairly uncommon but they are known to occur.

03:35 Now, we´ve discussed the grooves.

03:39 Let´s move inside and talk about the fates of the pharyngeal pouches.

03:44 The first pouch grows outward.

03:47 Now, remember, the first pouch is on the inside and it´s located between the first pharyngeal arch and the second pharyngeal arch.

03:54 It´s going to extend towards the first groove and it´s actually gonna develop into the middle ear and the auditory tube.

04:01 The connection between the middle ear and the pharynx.

04:04 The second pharyngeal pouch only deepens a bit.

04:08 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.

04:16 They grow outward but they develop a ventral and dorsal little division.

04:23 So it actually splits in two as it deepens.

04:26 As development proceeds, the first groove and the first pouch come very close together until only a thin membrane separates them.

04:35 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.

04:48 The second pharyngeal pouch deepened a bit but it just serves as the bed for the tonsil.

04:55 So your palatine tonsil has lymphatic tissue migrate in and hang out in the little divot that the second pharyngeal pouch created.

05:04 So the pouch does not make the tonsil, it just serves as the bed for those lymphatic cells that migrate in.

05:10 Now, the third and fourth pharyngeal pouches are where things really get interesting.

05:15 Those little divisions, the ventral and dorsal ones deepen and extend even further into the underlying mesenchyme and they´re gonna become glandular structures.

05:25 The third pharyngeal pouch is gonna create the inferior parathyroid gland and the thymus.

05:32 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.

05:43 The thymus gland is involved with maturation of T-cells and our immune response.

05:48 Now, the fourth pharyngeal pouch also produces a parathyroid gland.

05:53 In this case, the superior parathyroid gland and something called the ultimopharyngeal body.

05:59 Also sometimes called the ultimobranchial body.

06:02 So if you see that term, they really just mean the same thing.

06:05 This structure is going to give rise to C-cells that are located in the thyroid gland.

06:10 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.

06:23 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.

06:32 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.

06:42 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.

07:04 With the parathyroid glands stuck into its posterior substance of the lateral lobes of the thyroid gland.

07:12 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.

07:26 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.

07:43 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.

07:54 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.

08:05 Now, one very commonly described syndrome involving the thymus gland is DiGeorge syndrome.

08:10 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.

08:31 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.

08:51 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.

09:11 Thank you very much and I´ll see you in the next talk.