The importance of Embryology is highlighted in the fact that a sound anatomical knowledge cannot be acquired without knowing the origin of the postnatal structures. This article aims to provide the reader with a better understanding of the main structures of the head and neck and their prenatal development. Such information is essential for future medical professionals, especially Dentists and ENT specialists (Ear-Nose-Throat).
During Embryological development the Pharyngeal Arches appear in the fourth week as c-shaped rolls in a stack, separated by clefts. There are initially six arches, but the fifth arch regresses before development is complete. Pharyngeal pouches are developed from the endoderm and they open towards the Pharyngeal Clefts. Four out of the five facial swellings derive from the Pharyngeal Arches. These include the bilateral Maxillary and Mandibular processes and exclude the singular Frontonasal prominence. The external surface is composed of Ectoderm and the internal surface is lined with Endoderm. In the centre, the components include Lateral Plate Mesoderm, Paraxial Mesoderm and The Neural Crest. The Neural Crest tissue provides for the development of skeletal components, whereas the musculature develops from the Mesoderm. A cranial nerve is assigned to each individual Arch and its branches migrate with the muscles, to keep the contact for future innervation.
Anatomy and Supply
Now each Arch will be listed in order, including details of the musculature derived from the Mesoderm, the skeletal structures that develop from the neural crest, the cartilage present, the connective tissue structures and the nerve assigned to that particular arch.
- Arch 1: Develops into the bilateral maxillary and mandibular processes. Within this arch the mesodermal cells create the masseter muscle, the temporalis muscle, the lateral Pterygoid muscle, the medial Pterygoid muscle, the Mylohyoid muscle, the anterior body of the digastric muscle, the tensor tympani muscle and the tensor Veli Palatini muscle. The neural crest contributes the following skeletal structures: the maxilla, the squamous portion of the temporal bone, the zygoma (both bone and arch), the mandible, the malleus and the incus. Cartilages include Meckel’s Cartilage (degenerates in adulthood) and connective tissue exists in the form of the sphenomandibular ligament and the anterior ligament of the malleus. The nerve that innervates this arch is the Trigeminal nerve (CN V).
- Arch 2: The mesoderm produces the muscles of facial expression, the posterior body of the digastric muscle, the Stylohyoid muscle and the Stapedius muscle. Bones from the neural crest include the lesser cornu of the hyoid, the superior part of the hyoid body, the styloid process and the stapes. Reichert’s cartilage is the only one present in this arch. The Stylohyoid ligament and the connective tissue of the tonsil make up the connective tissue structures for this arch. The entire arch is innervated by the Facial Nerve (CN VII).
- Arch 3: The third arch produces the Stylopharyngeus muscle with its mesoderm. The bones that grow from the neural crest are the greater cornu of the hyoid and the inferior part of the hyoid body. There are no cartilaginous structures in the third pharyngeal arch. Connective tissue is seen in the thymus and inferior parathyroid. The glossopharyngeal nerve (CN IX) innervates the third pharyngeal arch.
- Arch 4: the mesoderm of the 4th arch contributes the uvula muscle, the Levator Veli Palatini muscle, the Palatopharyngeus muscle, the Palatoglossus muscle, the superior, middle and inferior pharyngeal constrictor muscles, the Salpingopharyngeus muscle and the Cricothyroid muscle. There are no skeletal structures in the 4th pharyngeal arch. The thyroid cartilage derives from the lateral plate mesoderm and the other cartilaginous structure is the epiglottis. The connective tissue is situated in the superior parathyroid and the thyroid gland. These structures are innervated by the Vagus Nerve (CN X).
- Arch 5: This arch is not worth mentioning since it forms with the others in the very early stages and then regresses almost immediately. When visible, it looks like a very small appendage between the fourth and sixth arches.
- Arch 6: Mesodermal musculature includes the thyroarytenoid vocalis muscle, the lateral, oblique, transverse and posterior arytenoid muscles, the aryepiglottis muscle and the thyroepiglottis muscle. There are no bones in this pharyngeal arch. Cartilages include the arytenoid, cricoid, cuneiform and corniculate which is form the lateral plate mesoderm. There is no connective tissue in this area. Like the fourth arch, this arch is innervated by the Vagus Nerve (CN X).