Pharyngeal arches, pouches, and clefts

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.

Each pharyngeal arch, with its associated pouch and cleft, carries its own cranial nerve that innervates the structures that develop 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.