AL1 Dev of Pharyngeal Arches, Pouches & Clefts (hold)

• Normal development of the pharyngeal arches, pouches, and clefts


  1. Development of the head and neck is formed by the pharyngeal (branchial) arches

  4. Pharyngeal arches contribute to the formation of the neck and face

  5. These arches appear in the fourth and fifth weeks of development and contribute to the characteristic external appearance of the embryo

  6. Initially, they consist of bars of mesenchymal tissue (arches) separated by deep pockets (pouches) and grooves (clefts)
• Pharyngeal arches

  Bars of mesenchymal tissue separated by deep pockets (pouches) and grooves (clefts)
• Pharyngeal arches

  • Each arch has a core of mesenchymal tissue, an outer covering of surface ectoderm, and an inner lining by endodermal epithelium
  • Each arch has its own artery, nerve and cartilages
• Initially there are 6 pharyngeal arches, but finally only 5 remain due to the disappearance of the 5th arch which is transient
• Pharyngeal pouches

  Deep pockets formed between the pharyngeal arches
• Pharyngeal clefts

  Deep grooves formed between the pharyngeal arches
• Derivatives of the pharyngeal pouches

  • 1st pouch: Tympanic cavity (middle ear cavity), Auditory tube (Eustachian tube), Tympanic membrane (endodermal lining)
  • 2nd pouch: Palatine tonsil, Tonsillar fossa
  • 3rd pouch: Dorsal part - Inferior parathyroid gland, Ventral part - Thymus gland
  • 4th pouch: Dorsal part - Superior parathyroid gland, Ventral part - Parafollicular/C cells of thyroid gland
• Derivatives of the 1st pharyngeal arch

  • Nerve: Maxillary division and Mandibular division of CN V (Trigeminal)
  • Muscles: Muscles of mastication (Temporalis, Masseter, Medial pterygoid, Lateral pterygoid, Mylohyoid, Digastric (anterior belly), Tensor palatini, Tensor tympani)
  • Skeletal components: Premaxilla, Maxilla, Zygomatic, Mandible, Malleus, Incus, Anterior ligament of malleus, Spheno-mandibular ligament
• Derivatives of the 2nd pharyngeal arch

  • Nerve: CN VII (Facial)
  • Muscles: Muscles of facial expression (Frontalis, Orbicularis oris, Orbicularis oculi, Platysma, Buccinator, Digastric (posterior belly), Stapedius, Stylohyoid)
  • Skeletal components: Stapes, Styloid process, Stylohyoid ligament, Hyoid (lesser horn and upper part of body)
• Derivatives of the 3rd pharyngeal arch

  • Nerve: CN IX (Glossopharyngeal)
  • Muscles: Stylopharyngeus
  • Skeletal components: Hyoid (greater horn and lower part of body)
• Derivatives of the 4th and 6th pharyngeal arches

  • Nerve: Superior laryngeal branch of CN X (Vagus), Recurrent laryngeal branch of CN X (Vagus)
  • muscle = cricothyroid, levator palatini
  • skeletal component = thyroid, cricoid
• The external appearance and typical features of the face, head and neck regions are due to the original (paraxial and lateral plate) mesoderm of the arches which form the muscles, and the migrating neural crest cells which form the skeleton of the face
• Derivatives of 1st pharyngeal cleft

  • External auditory meatus
  • Outer layer of tympanic membrane
• 2nd, 3rd, and 4th pharyngeal clefts are obliterated, may persist as cervical sinus
• Derivatives of 1st pharyngeal pouch

  • Tympanic cavity (middle ear cavity)
  • Auditory tube (Eustachian tube)
  • Tympanic membrane (endodermal lining)
• Derivatives of 2nd pharyngeal pouch

  • Palatine tonsil
  • Tonsillar fossa
• Derivatives of 3rd pharyngeal pouch


  • Dorsal part: Inferior parathyroid gland
  • Ventral part: Thymus gland
• Derivatives of 4th pharyngeal pouch

  • Dorsal part: Superior parathyroid gland
  • Ventral part (ultimobranchial body): Parafollicular (C) cells of thyroid gland
• Lateral cervical cysts are formed by unclosed cervical sinus (remnants of 2nd, 3rd, 4th clefts)
• Internal branchial fistulas are due to opening of cervical sinus towards lumen of pharynx
• External branchial fistulas are due to opening of cervical sinus towards outside
• Insufficient migration of neural crest cells into the first pharyngeal arch may give rise to severe craniofacial defects such as first arch syndrome
• craniofacial defect = Treacher Collins syndrome (mandibulofacial dysostosis) is characterized by malar hypoplasia, mandibular hypoplasia, and malformed external ears
• mesenchyme of the head region is derived from

  1. paraxial mesoderm
  2. lateral plate mesoderm
  3. neural crest (neuroectoderm)
• Original (paraxial and lateral plate) mesoderm of arches which form

  muscle of neck and head
• Migrating neural crest cells which form the

  skeleton of face
• Since the lining of the pharyngeal pouches is endodermal it will develop into glands:


  inferior parathyroid and thymus
• Derivatives of the inferior parathyroid and thymus gland

  1. Primordia of both glands lose contact with the pharyngeal wall2. The inferior parathyroid is carried to a position below the superior parathyroid, along with the descent of the thymus gland

  
• lateral cervical cyst

  a swelling on the lateral side of neck, along the anterior border of the sternocleidomastoid muscle, below the angle of jaw
• Ectopic thymic and parathyroid glands

  • Normally thymus and inferior parathyroid glands lose contact with the pharyngeal pouches and descend to a lower position.
  • During their descent: Thymus gland may remain in the neck. Inferior parathyroid gland may lie at the level of bifurcation of common carotid artery.