AssessmentsPharyngeal arches, pouches, and clefts
Pharyngeal arches, pouches, and clefts
USMLE® Step 1 style questions USMLE
USMLE® Step 2 style questions USMLE
A 29-year-old man comes to the office with a lump on the side of his neck. Last week he had a severe cold and the bump appeared and has grown steadily over the last few days. He denies any fevers, chills, night sweats, or weight changes. On physical examination there is a well-defined, mobile 2-cm nodule just anterior to the sternocleidomastoid muscle that does not move during swallowing. The rest of the exam is unremarkable. Which of the following is the most likely diagnosis?
Early in development, the embryo is a flat, disc-shaped organism made up of three layers of pluripotent cells called germ layers: an inner layer, called endoderm, a central layer, called mesoderm, and an outer layer, called ectoderm.
These three specialized cell layers give rise to all the organs and tissues in the body.
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 cranial (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 around weeks 4 and 5, when six little bars of mesoderm, the pharyngeal arches, sprout from the primitive pharynx.
The pharyngeal arches develop in a craniocaudal fashion—meaning they form at the head and continue developing towards the tail end of the fetus.
These paired, symmetrical bumps are numbered from 1 to 6—it’s important to note that the fifth arch either never forms, or it quickly regresses, so it doesn’t develop into any structures.
Between the five pharyngeal arches, four pharyngeal clefts form and cover the external part of the corresponding arch with ectoderm cells, while four pharyngeal pouches line the internal part of their corresponding arches 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.
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 temporal bones as well as the zygomatic bones or cheekbones. To remember that, you might think of Ziggy Stardust— who we can all agree had 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 (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; a lot of these structures help us make facial expressions.
In terms of 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, a tiny ear bone (the smallest bone in the body) which works with the malleus and incus to help 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 help us with swallowing.
Other muscles derived from this arch are the tiny stapedius muscles, which anchor the stapes of the ears.
Third pharyngeal arch structures are innervated by the glossopharyngeal nerve.
There’s actually only two structures that originate from third arch: the rest of the hyoid bone, and one muscle of the throat (the stylopharyngeus) which helps with swallowing.
Now, remember, because the fifth arch does not form anything, our last pharyngeal arches are the fourth and sixth.
These are both innervated by branches of the vagus nerve: the superior laryngeal branch innervates the fourth arch’s structures, and the recurrent laryngeal branch innervates the sixth arch’s structures.
The fourth and sixth arches don’t form any bones, but they do work together to form the laryngeal cartilages.
The fourth pharyngeal arch gives rise to muscles in the mouth, pharynx, and larynx: the levator palatini, which prevents food from entering the respiratory tract while we swallow; the pharyngeal constrictors, which squeeze food down the esophagus; and the cricothyroid muscle, which tenses the vocal cords to produce sound.
The sixth arch gives rise to the rest of the intrinsic muscles of the larynx that help us speak.
Three of the pharyngeal arches work together to form the tongue, which speaks to the complexity of this unique, muscular organ.
The anterior two-thirds of the tongue start out 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; these arches form most of the structures in the pharynx, which is where the posterior portion of the tongue is.
When it comes to structures formed by the pharyngeal arches, there’s a lot to remember, so we put together an extensive mnemonic to help you recall this important information more easily. Here, we’ve got five different characters at the circus, who represent the five pharyngeal arches.