The pharyngeal arches are temporary structures that appear during early human embryonic development. These structures are bilateral tissue swellings that emerge from the head region of the neural crest. They are precursors for many structures in the head and neck. The first pharyngeal arch is a significant structure, playing a substantial role in the formation of various parts of the face, jaw, and ear.
How the First Pharyngeal Arch Forms
The first pharyngeal arch, also known as the mandibular arch, is the first of six pharyngeal arches to develop. It appears during the fourth week of embryonic development, as the embryo is rapidly forming its basic body plan. These arches are found as a series of bulges on the lateral surface of the embryonic head.
Each pharyngeal arch consists of a core of mesenchyme, which is a type of embryonic connective tissue. This core is covered externally by ectoderm and lined internally by endoderm. The mesenchyme itself originates from both the mesoderm and neural crest cells, which migrate into the developing head and neck regions. The first arch is located just below the developing lens placode and divides into two distinct parts: a dorsal maxillary process and a ventral mandibular process.
What the First Pharyngeal Arch Becomes
The first pharyngeal arch undergoes extensive differentiation to form a wide array of structures in the head and neck. Its derivatives include various skeletal components, muscles, nerves, and blood vessels. These structures are organized into specific categories based on their tissue type.
Skeletal and Cartilaginous Derivatives
The first pharyngeal arch contributes to several bones of the face and middle ear. The maxillary process gives rise to the maxilla (upper jaw bone), the zygomatic bone (cheekbone), and portions of the palatine bone and the squamous part of the temporal bone. The mandibular process, containing Meckel’s cartilage, is responsible for forming the mandible (lower jaw bone), which molds around this cartilage through membranous ossification. Most of Meckel’s cartilage degenerates, but its dorsal portion persists to form the malleus and incus, two small bones of the middle ear, along with the anterior ligament of the malleus and the sphenomandibular ligament.
Muscular Derivatives
Muscles derived from the first pharyngeal arch are primarily involved in chewing and other facial movements. These include the four muscles of mastication: the masseter, temporalis, medial pterygoid, and lateral pterygoid muscles. Other muscles originating from this arch are the mylohyoid and the anterior belly of the digastric, both involved in floor of mouth movements. The tensor tympani, which helps protect the inner ear from loud noises, and the tensor veli palatini, involved in soft palate movement, also develop from the first arch.
Nerve Supply
The nerve supply to structures derived from the first pharyngeal arch comes from the trigeminal nerve (Cranial Nerve V). Specifically, its maxillary (V2) and mandibular (V3) branches innervate both the muscles and sensory areas. The mandibular branch provides motor innervation to all the muscles originating from the first arch, including the muscles of mastication. It also provides sensory innervation to the skin of the face, the lining of the mouth and nose, and general sensation to the front two-thirds of the tongue.
Vascular Supply
The arterial supply to the first pharyngeal arch is provided by the first aortic arch artery. This artery partially persists to form the terminal portion of the maxillary artery, which is a branch of the external carotid artery. This vascular component ensures blood flow to the developing structures of the arch.
When Development Goes Awry
Errors in the development of the first pharyngeal arch can lead to a range of congenital conditions, often affecting facial structures. These conditions frequently result from issues with the migration or formation of neural crest cells that contribute to the arch. The manifestations of these syndromes directly relate to the structures that normally arise from the first arch.
One such condition is Treacher Collins Syndrome (TCS), also known as mandibulofacial dysostosis. This autosomal dominant disorder is often caused by mutations in the TCOF1 gene, leading to a reduced number of cranial neural crest cells migrating to the first and second pharyngeal arches. Individuals with TCS typically present with underdeveloped facial bones, particularly the jaw and cheekbones, and malformed ears, reflecting the arch’s role in these structures.
Pierre Robin Sequence is another condition where first pharyngeal arch development is affected. It is characterized by a triad of micrognathia (an unusually small lower jaw), glossoptosis (a tongue that falls backward), and often a cleft palate. The small mandible, a derivative of the first arch, can cause breathing difficulties and feeding problems in affected infants.
Goldenhar Syndrome, also known as oculo-auriculo-vertebral spectrum (OAVS) or hemifacial microsomia, involves the improper development of the first and second pharyngeal arches. This condition frequently results in asymmetric facial development, where one side of the face is less developed than the other. Manifestations can include ear anomalies, such as microtia (underdeveloped outer ear), and issues with the jaw and facial bones on the affected side.