Pharyngeal arches, pouches, and clefts

Definitions & Key takeaways

Pharyngeal arches, pouches, and clefts are structures that develop in the embryonic stage of human development and are involved in the formation of various head and neck structures, including the face and ears. The pharyngeal arches are derived from mesoderm and give rise to many of the bones and muscles in the head and neck. The pharyngeal clefts derive from the ectoderm, forming structures in the ear canals. The pharyngeal pouches arise from endoderm cells, and these form parts of the ears, as well as the early tonsils, and many portions of the parathyroid glands and thyroid.

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Early in development, the embryo is a flat, disc-shaped organism made up of three layers of pluripotent cells called germ layers, which give rise to all the organs and tissues in the body: an inner layer, called endoderm, a central layer, called mesoderm, and an outer layer, called ectoderm.
By week 4 of development, the embryo takes on a more recognizably “human” form—but to be honest, it still looks more like a shrimp than a baby.
At the head end of this little shrimp-like creature, a set of structures called the pharyngeal apparatus begins to develop, consisting of pharyngeal arches, clefts, and pouches.
The pharyngeal apparatus starts forming when six little bars of mesoderm, the pharyngeal arches, sprout from the primitive pharynx in a craniocaudal fashion - going from head to tail.
These arches are paired, symmetrical bumps that form during weeks 4 and 5. The arches are numbered from 1 to 6- the 5th either never forms, or it quickly regresses, so it doesn’t develop into any structures.
Between the 5 pharyngeal arches, 4 pharyngeal clefts form and cover the external part of the arch with ectoderm cells while 4 pharyngeal pouches line the internal part of the arch with endoderm.
The components of the pharyngeal apparatus develop into various head and neck structures, and sometimes multiple arches join together to give rise to a single structure.
Each pharyngeal arch, with its associated pouch and cleft, carries its own cranial nerve that innervates the structures forming from that arch.
The first pharyngeal arch is mainly associated with everything we need to chew. Structures from this arch are innervated by the trigeminal nerve - more specifically, its mandibular branch.
In terms of bones, it gives rise to the maxilla, which forms the upper jaw, and the mandible which forms the lower jaw. Two small portions of the mandible will give rise to the incus and the malleus bones of the middle ear, which resemble an anvil and a hammer and transmit sound vibrations from the eardrums.
The first pharyngeal arch also forms part of the temoporal bones as well as the zygomatic bones or cheekbones. To remember, you might think of Ziggy Stardust who has striking cheekbones.
Muscles that come from the first pharyngeal arch include muscles that help us chew—the temporalis, masseter, and pterygoid muscles - a muscle that blocks out noises from chewing - the tensor tympani - and some of the muscles that help us swallow - such as the tensor veli palatini, the mylohyoid muscles and the anterior belly of the digastric.
The posterior belly of the digastric will be formed by the second arch - so there’s an example of pharyngeal arch teamwork.
The second pharyngeal arch forms structures that will be innervated by the facial nerve and a lot of these structures take part in forming facial expression.
For the bones, we have the hyoid bone, specifically the lesser horns and the upper portion - and the styloid process of the temporal bone.
The second arch also forms the stapes, the tiniest bones in the body which works with the malleus and incus to helps transmit sound to the inner ear.
Muscles from the second pharyngeal arch mainly control facial movement and expression. Some second arch muscles like the posterior belly of the digastric and the stylohyoid muscle also helps with swallowing.
Other muscles derived from this arch are the tiny stapedius muscle, which anchors the stapes of the ears. Third pharyngeal arch structures are innervated by the glossopharyngeal nerve.
There’s actually only two of them: the rest of the hyoid bone and one muscle of the throat, the stylopharyngeus, that helps with swallowing.
Now, remember, because arch five does not form anything, our last pharyngeal arches are four and six. They are both innervated by branches of the vagus nerve: the superior laryngeal branch innervates fourth arch structures, and the recurrent laryngeal branch innervates sixth arch structures.
The two arches don’t form any bones, but they work together to form the laryngeal cartilages. Muscle-wise, the fourth pharyngeal arch gives rise to the levator palatini, which prevents food from entering the respiratory tract while we swallow; and the pharyngeal constrictors, which squeeze food down the esophagus, both of these are muscles in the mouth and pharynx.
The cricothyroid muscle, which tenses the vocal cords to produce sound. The sixth arch gives rise to all the rest of the intrinsic muscles of the larynx that help us speak.
So the cricothyroid of the fourth arch, as well as the 6th arch helps form the muscles of the larynx. The tongue is an structure that takes three arches to form.
The anterior two-thirds starts as a bud from the floor of the first arch. We can remember this because when we chew with structures formed by the first arch, we might end up biting the anterior portion of the tongue.
The posterior one-third comes from buds of the third and fourth arches and these arches form most of the structures in the pharynx which is where the posterior portion of the tongue is.
To memorize the structures in the pharyngeal arch, we will use 5 different characters at the circus to represent the 5 arches.
Lets imagine a little boy named Billy happily chewing gum while visiting the circus. This is the first character so arch 1, and he’s chewing gum so muscles and bones of the jaw come from arch one.
His name is Billy and it’s on the front of his shirt, reminding us of the anterior belly of the digastric. And his favorite game is the strength machine which has a giant hammer and a giant anvil at the bottom, representing the malleus and incus.
Billy smashes the anvil with the hammer, and his bubble gum pops which startles him so he tenses up, reminding us of tensor tympani and tensor veli palatini.
Next, Billy meets a clown with a big frown who’s also named Billy, what a coincidence! So this is the second character so arch 2, and his frown reminds us of muscles of facial expression.
The clown’s name is also Billy, but it’s printed on the back of his shirt, reminding us of the posterior belly of the digastric.
Billy the clown works part time in the rodeo, and his foot is still stuck in a Stirrup, reminding us of the stapes and stapedius muscle, and Billy the clown is carrying a bunch of horseshoes, reminding us of the horseshoe shaped hyoid bone.
Billy heads over to the circus’ main attraction, two giraffes, one with three spots and one with four spots, both with long beautiful necks.
So these remind us that arch 3 and arch 4 give rise to the pharynx. The 3 spotted giraffe likes to wear accessories like horseshoes, reminding us that arch 3 also contributes to the horseshoe shaped hyoid bone.
The three spotted giraffe also like to wear a stylish scarf representing the stylopharyngeus muscle. Meanwhile, the 4 spotted giraffe is busy eating leaves, reminding us that arch 4 gives rise to the muscles that help move food down the esophagus.
Suddenly, the giraffe starts choking because something went down its airpipe! That’s because it has a problem with the levator palatini muscle that normally prevents food from going down the wrong tube, so it ends up with a cricket in its larynx which represent the cricothyroid muscles.
The master of ceremonies that day is a 6 year old mime, very talented! So that reminds us that arch 6 gives rise to the muscles in the larynx that produce sound.
The exception is the cricothyroid that’s stuck in the 4 spotted giraffe. They did a surgery to remove the cricket and the 6-year old mime puts a long, hard cast around the giraffe’s neck representing the laryngeal cartilages which comes from arches 4 and 6.
Separating the pharyngeal arches on the outside of the embryo are four pharyngeal clefts, layered with ectoderm cells. Clefts 2-4 fade as the embryo grow while the first pharyngeal cleft works closely with the first pharyngeal pouch to form the ear.
The cleft being on the outside, gives rise to the external auditory meatus or ear canal and it also form the ear drums. The first pharyngeal pouch which is on the inside, gives rise to a long, thin cavity that expands to from the internal auditory meatus or middle ear.
A small portion of the cavity remains narrow, becoming the eustachian tube between the auditory canal and the nasopharynx.
Cells lining the second pharyngeal pouch multiply and migrate to form the primitive tonsils. The third pouch and fourth pouch are very similar because they both divide into a dorsal and ventral portion.
The dorsal portion of the third pouch becomes the inferior parathyroid gland while the dorsal portion of the fourth pouch becomes the superior parathyroid gland.
These glands create parathyroid hormones which help increase calcium levels in the blood. The ventral portion of the third pouch becomes the primitive thymus and will later descend down to the chest.
The ventral portion of the fourth pouch becomes the ultimopharyngeal body, a pouch like embryological structure that contains cells which differentiate into parafollicular or C-cells of the thyroid.
C-cell produce calcitonin which lowers blood calcium. The thyroid develops from the endoderm at the base of the tongue independent of the pharyngeal apparatus, it will descend down the neck and the parathyroid glands will latch onto the thyroid as it passes by while cells from the ultimopharyngeal body will also migrate into the thyroid as it continues down to rest at the base of the neck.
All right, as a quick recap… The pharyngeal arches derive from mesoderm, giving rise to the many of the bones and muscles in the head and neck.
The pharyngeal clefts derive from ectoderm, forming structures in the ear canals. The pharyngeal pouches arise from endoderm cells, and these form parts of the ears, as well as the early tonsils and many portions of the parathyroid glands and thyroid.
Pharyngeal arches, pouches, and clefts: Video | Osmosis