The Pharyngeal Arches – Part 1

  • There are 5 pharyngeal arches, numbered 1 – 4 and then 6.

(There is NO 5th pharyngeal arch.)

  • They comprise:
    • Aortic arches, the arterial connections between the ventral and dorsal aortae.
    • Skeletal structures (derived from neural crest cells).
    • Muscle (derived from mesoderm).
    • Cranial nerves (derived from neural crest cells).

THE PHARYNGEAL ARCHES AND THE NEURAL CREST CELL MIGRATIONS TO FORM CNS 5, 7, 9, AND 10

We draw the differentiated neural tube from cranial to caudal:

  • Telencephalon
  • Diencephalon
  • Mesencephalon
  • Metencephalon
  • Myelencephalon

And we include the caudal neural tube

  • The notochord induces the overlying ectoderm to differentiate into the neural tube.
  • There are 5 pharyngeal arches, from cranial to caudal: 1, 2, 3, 4, (skip 5) and then 6 [No 5th Pharyngeal Arch exists!]

Neural crest cells migrate into the pharyngeal arches and to form the pharyngeal arch cranial nerves.

  • CN 5 (the trigeminal nerve) migrates into arch 1
  • CN 7 (the facial nerve) migrates into arch 2
  • CN 9 (the glossopharyngeal nerve) migrates into arch 3
  • CN 10 (the vagus nerve) migrates into arches 4 and 6 – the superior laryngeal branch lies within the 4th pharyngeal arch and the recurrent laryngeal branch lies within the 6th pharyngeal arch.

In addition to the cranial nerves being a part of this migration, so are the skeletal elements.

  • The mandibular prominence forms pharyngeal arch 1 (if we know CN 5’s role in mastication, this will help us remember the association between this arch and nerve).
  • Although the maxillary prominence is sometimes listed as a portion of pharyngeal arch 1, indicate that it actually lies rostral to the 1st arch.

Key placodes (which are areas of thickened surface ectoderm) derive CNs 1, 2, and 8 (the solely sensory set of CNs), from cranial to caudal.

  • At the nasal prominence, lies the olfactory placode, which derives the olfactory epithelium and olfactory nerve (CN 1).
  • The optic placode forms the optic nerve (CN 2); it originates from the diencephalon.
  • The otic placode forms the vestibulocochlear nerve (CN 8); it originates from the hindbrain.

THE PHARNGYEAL APPARATUS (AKA THE PHARYNGEAL REGION): THE PHARYNGEAL POUCHES AND AORTIC ARCHES.

Whereas the neural tube lies dorsal to the notochord, the structures we’ll focus on here (the vasculature and pharyngeal apparatus) lie ventral to it.

  • The long endodermal tube follows the cephalic bend ventral to the notochord.
    • Cranially, lies the pharynx.
    • Caudally, label the esophagus.
  • The trachea branches from the endodermal tube anterior to the esophagus.

4 pharyngeal pouches lie along the endoderm

We specify that the 1st pharyngeal pouch lies posterior to the 1st pharyngeal arch.
  • The pouches are outpouchings of endoderm that fill the pharyngeal grooves; we’ll understand this anatomy better in part 2 of our diagram in which we draw the pharyngeal apparatus in coronal view.

Arterial vasculature

  • Each pharyngeal arch has an aortic arch that runs within it.
  • From the heart emanates the truncus arteriosus, aortic sac, and the ventral aorta.
  • The dorsal aorta bifurcates to become the bilateral internal carotid arteries, cranially – they form the primary supply of blood to the brain (the anterior 2/3rds of the brain’s vascular supply). For reference, the posterior blood supply to the brain comes from the basilar artery, which is supplied by the vertebral arteries.
  • Connect the ventral and dorsal aortae with the aortic arches that pass in between the pharyngeal pouches and specify the 1st aortic arch (these are sometimes referred to simply as the arch arteries) – they connect the dorsal and ventral aortae.

THE PITUITARY GLAND

  • Rathke’s pouch is an ectodermal placode along the roof of the stomodeum (the site of the future mouth – the cranial opening of the pharyngeal apparatus). Rathke’s pouch stretches towards the floor of the 3rd ventricle (the infundibulum). Later, it disconnects from the stomodeum and its stalk regresses: ultimately, forming the anterior pituitary gland. And the infundibulum descends and develops into the posterior pituitary gland.
    • Clinical Correlation: Craniopharyngioma
These fascinating embryological migrations help us to remember that the pituitary gland is acutely in touch with the external environment and works to keep our body in physiological homeostasis.

THE THYROID GLAND

  • The thyroid primordium lies in between the 1st and 2nd pharyngeal pouches, along the ventral surface of the pharyngeal apparatus, draw. It forms at the apex (the ventral tip) of the foramen cecum.
  • The thyroid primordium develops into the thyroid gland, which descends within the thyroglossal duct (which quickly breaks down) and then migrates beneath the thyroid cartilage to its ultimate anatomical site: beneath the cricoid cartilage.

Clinical Correlation –

The cricoid cartilage is an important anatomical landmark when palpating for a thyroid goiter!

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