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.

This quiz covers the development of pharyngeal arches and pouches in the formation of the head and neck, a key topic in histology and embryology. Learn about the typical features of this process and more.