Pharyngeal Arches, Pouch and Cleft

General Information

• The pharyngeal arches, pouches, and clefts are paired structures that appear during the 4th week of human embryonic development
•  Branchial Arches form Clefts externally and Pouches internally (separated by pharyngeal membranes)
  • Pouches lined by endoderm 
  • Arch derivatives lined by mesoderm 
  • Cleft derivatives lined by ectoderm
• Each arch associated with:
  • Cartilaginous bar
  • Carotid artery derivative
  • Cranial nerve
  • Vein

Pharyngeal Arches

• The pharyngeal arches are five paired structures (1st, 2nd, 3rd, 4th, and 6th) visible externally as bulges in the developing embryo.
• Each arch contains:
  • Mesodermal core: Develops into muscles and blood vessels.
  • Neural crest cells: Contribute to bones, cartilage, and connective tissues.
  • Cranial nerve: Specific to each arch.
  • Aortic arch artery: Forms part of the vascular system.

Key Derivatives:

1. 1st Arch (Mandibular):
   • Bones: Maxilla, mandible, malleus, and incus.
   • Muscles: Mastication muscles (e.g., temporalis, masseter), anterior belly of digastric.
   • Nerve: Mandibular division of the Trigeminal nerve (CN V3).
2. 2nd Arch (Hyoid):
   • Bones: Stapes, styloid process, lesser horn of hyoid.
   • Muscles: Facial expression muscles, posterior belly of digastric.
   • Nerve: Facial Nerve (CN VII).
3. 3rd Arch:
   • Bones: Greater horn of hyoid.
   • Muscles: Stylopharyngeus.
   • Nerve: Glossopharyngeal nerve (CN IX).
4. 4th Arch:
   • Cartilage: Thyroid and cricoid cartilage.
   • Muscles: Pharyngeal constrictors, cricothyroid.
   • Nerve: Superior laryngeal branch of vagus nerve (CN X).
5. 6th Arch:
   • Cartilage: Laryngeal cartilages.
   • Muscles: Intrinsic laryngeal muscles (except cricothyroid).
   • Nerve: Recurrent laryngeal branch of vagus nerve (CN X).

Pharyngeal Pouch

The pouches are internal structures made of endoderm, located between the pharyngeal arches.

1. 1st Pouch:
   • Forms the Eustachian tube and middle ear cavity.
2. 2nd Pouch:
   • Forms the palatine tonsils.
3. 3rd Pouch:
   • Dorsal part: Inferior parathyroid glands.
   • Ventral part: Thymus.
4. 4th Pouch:
   • Dorsal part: Superior parathyroid glands.
   • Ventral part: Ultimopharyngeal body (forms parafollicular cells of the thyroid).

Pouch	Location	Embryonic Structure	Adult Structure
1	Opposite the 1st pharyngeal cleft, separated by the 1st pharyngeal membrane	Tubotympanic recess	- Epithelium of the (pharyngotympanic auditory) tube - Tympanic cavity
2	Opposite the 2nd pharyngeal cleft, separated by the 2nd pharyngeal membrane	Primordial palatine tonsils	- Tonsillar (sinus fossa) - Epithelium of the palatine tonsil
3	Opposite the 3rd pharyngeal cleft, separated by the 3rd pharyngeal membrane	- Divides into a dorsal and a ventral part - Dorsal part migrates inferiorly toward the thorax	- Inferior parathyroid gland (from the dorsal part) - Thymus (from the ventral part)
4	Opposite the 4th pharyngeal cleft, separated by the 4th pharyngeal membrane	- Divides into a dorsal and a ventral part - Ventral part is invaded by neural crest to form the parafollicular or C cells	- Superior parathyroid gland (from the dorsal part) - Ultimobranchial body (from the ventral part)

Pharyngeal Cleft

1. 1st Cleft:
   • Forms the external auditory meatus.
2. 2nd–4th Clefts:
   • These are overgrown by the 2nd arch, forming the cervical sinus, which later obliterates. Failure to obliterate can lead to branchial cysts.

Cleft	Location	Adult Structure
1	A groove between the 1st and 2nd pharyngeal arches	External acoustic meatus
2	A groove between the 2nd and 3rd pharyngeal arches	Obliterated cervical sinus by the 2nd pharyngeal arch, which grows over the clefts
3	A groove between the 3rd and 4th pharyngeal arches	”
4	A groove between the 4th and 6th pharyngeal arches	”

Summary