Development of Pharyngeal Arches and Pouches in Head and Neck

Development of Pharyngeal Arches

• Pharyngeal or branchial arches form in the 4th and 5th weeks of development, characterized by bars of mesenchymal tissue separated by clefts (pharyngeal clefts).
• Each arch consists of a core of mesenchyme covered by surface ectoderm and epithelium of endodermal origin.
• The core contains mesenchyme derived from paraxial and lateral plate mesoderm and neural crest cells.
• Each arch has its own muscular components, cranial nerve, and arterial component.

First Pharyngeal Arch

• Dorsal portion forms the maxillary process.
• Ventral portion forms the mandibular process, which contains Meckel's cartilage.
• Meckel's cartilage disappears except for two portions at its dorsal end, forming the incus and malleus.
• Mesenchyme of the maxillary process gives rise to the premaxilla, maxilla, and zygomatic bone.
• Mandible is formed by membranous ossification of mesenchymal tissue.

Second Pharyngeal Arch (Hyoid Arch)

• Cartilage gives rise to the stapes, styloid process of the temporal bone, stylohyoid ligament, and lesser horn of the hyoid bone.

Third Pharyngeal Arch

• Cartilage gives rise to the lower part of the body of the hyoid bone and the greater horn of the hyoid bone.

Fourth and Sixth Pharyngeal Arches

• Cartilaginous components fuse to form the cartilages of the larynx (thyroid, cricoid, arytenoid, corniculate, and cuneiform).

Pharyngeal Pouches

• Human embryo has 4 pairs of pharyngeal pouches, with the 5th pair being absent or rudimentary.
• Epithelial endodermal lining of the pouches gives rise to a number of organs.

First Pharyngeal Pouch

• Forms a stalk-like diverticulum, the tubotympanic recess, which comes in contact with the epithelial lining of the 1st pharyngeal cleft (future external auditory meatus).
• Distal portion widens to form the middle ear cavity, while the proximal portion remains narrow, forming the auditory (eustachian) tube.

Second Pharyngeal Pouch

• Epithelial cells proliferate and form buds that penetrate into the surrounding mesenchyme, forming the palatine tonsil.

Clinical Correlates

• Ankyloglossia (tongue-tie): a condition where the tongue is not freed from the floor of the mouth.
• Branchial fistula: occurs when the 2nd pharyngeal arch fails to grow caudally over the 3rd and 4th arches, leaving remnants of the 2nd and 3rd clefts in contact with the surface by a narrow canal.
• Lateral cervical cyst: a remnant of the cervical sinus, located just below the angle of the jaw.
• Palatine tonsil: formed from the 2nd pharyngeal pouch.

Face Development

• Facial prominences consisting primarily of neural crest-derived mesenchyme appear at the end of the 4th week.
• The dorsal part of the 1st cleft penetrates the underlying mesenchyme and gives rise to the external auditory meatus.
• The epithelial lining forms the eardrum.

Thymus and Parathyroid Glands

• Third pharyngeal pouch contains dorsal and ventral wing, with the epithelium of the dorsal wing forming the inferior parathyroid gland and the ventral wing forming part of the thymus.
• Fourth pharyngeal pouch: epithelium of the dorsal wing forms the superior parathyroid gland, while the ventral part develops into the ultimobranchial body, which is later incorporated into the thyroid gland.
• Ultimobranchial body gives rise to the parafollicular (C cells), which differentiate from neural crest cells.