7. Differentiation of the pharyngeal arches. Malformations

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Learning objectives

• What are the pharyngeal arches?
• What are the derivatives of the first pharyngeal arch?
• What are the derivatives of the second pharyngeal arch?
• What are the derivatives of the third pharyngeal arch?
• What are the derivatives of the fourth pharyngeal arch?
• What are the derivatives of the sixth pharyngeal arch?
• What are the first arch syndromes?

Pharyngeal arches

During the fourth week the pharyngeal apparatus develops. The pharyngeal apparatus consists of six pharyngeal arches, each of which are covered on the outside by ectoderm and on the inside by the endoderm. The outside ectoderm lining forms the pharyngeal clefts, while the inside endoderm lining forms the pharyngeal pouches. The pharyngeal arches themselves consist of mesenchyme, which will give rise to arteries (aortic arches), muscles, cartilage, and bones. The mesenchyme also carries a cranial nerve or a branch of one, which will innervate the muscles which arise from the same arch.

There are six pharyngeal arches, of which only five give rise to permanent structures; the fifth pharyngeal arch obliterates completely. The pharyngeal arches appear in a cranial-to-caudal sequence, meaning that the most cranial arch is present first and the most caudal one last. This also means that not all arches are present simultaneously.

First pharyngeal arch

The first pharyngeal arch, also known as the mandibular arch, carries the mandibular branch of the trigeminal nerve. It gives rise to the muscles of mastication, the mylohyoid muscle, the anterior belly of the digastric muscle, the tensor tympani muscle, and the tensor veli palatini muscle. The first aortic arch gives rise to the maxillary artery.

This arch also gives rise to the mandibular and maxillary processes or prominences. The mandibular prominence gives rise to the mandible as well as the incus and malleus (through the Meckel cartilage). The maxillary prominence gives rise to the maxilla, zygomatic, and the squamous part of the temporal bone.

Second pharyngeal arch

The second pharyngeal arch, also known as the hyoid arch, carries the facial nerve. It gives rise to the muscles of facial expression, the posterior belly of the digastric muscle, the stylohyoid muscle, and the stapedius muscle. The second aortic arch gives rise to the hyoid and stapedial arteries.

This arch gives rise to the Reichert cartilage, which gives rise to the stapes, styloid process of the temporal bone, and part of the hyoid bone.

Third pharyngeal arch

The third pharyngeal arch carries the glossopharyngeal nerve. It gives rise to the stylopharyngeus muscles. The third aortic arch gives rise to the common carotid and part of the internal carotid.

It also gives rise to the rest of the hyoid bone.

Fourth pharyngeal arch

The fourth pharyngeal arch carries the superior laryngeal nerve, a branch of vagus. It gives rise to the cricothyroid muscle, the levator veli palatini, and the constrictor muscles of the pharynx.

Cartilaginous components of the fourth and sixth pharyngeal arches fuse to form the thyroid, and the cricoid, arytenoid, corniculate, and cuneiform cartilages of the larynx.

The left side of the fourth aortic arch gives rise to the aortic arch from the left common carotid to the left subclavian, while the right side gives rise to the proximal portion of the right subclavian artery.

Sixth pharyngeal arch

The sixth pharyngeal arch carries the recurrent laryngeal nerve, another branch of vagus. It gives rise to the intrinsic muscles of the larynx.

The left side of the sixth aortic arch gives rise to the left pulmonary artery and the ductus arteriosus, while the right side gives rise to the right pulmonary artery.

Malformations

First arch syndrome

First arch syndrome refers to a number of malformations which occur due to insufficient migration of neural crest cells into the first and second pharyngeal arches. The most common types are Treacher Collins syndrome and the Pierre Robin sequence.

Treacher Collins syndrome, also called mandibulofacial dysostosis, is a rare autosomal dominant syndrome characterised by hypoplasia of the maxilla, mandible, and zygomatic arch. It occurs due to a genetic mutation which impairs the migration of neural crest cells into the first and second pharyngeal arches.

Pierre Robin sequence, sometimes only known as Robin sequence, is characterised by small mandible, cleft palate, and a posteriorly placed tongue.

Summary

• What are the pharyngeal arches?
  • They are structures consisting of mesenchyme which will give rise to arteries, muscles, cartilage, bone
  • Each pharyngeal arch carries a cranial nerve or a branch of one
• What are the derivatives of the first pharyngeal arch?
  • Nerve: mandibular branch of trigeminal nerve
  • Arteries
    • Maxillary artery
  • Muscles
    • Muscles of mastication
    • Mylohyoid muscle
    • Anterior belly of digastric
    • Tensor tympani
    • Tensor veli palatini
  • Bones and cartilages
    • Meckel cartilage -> incus, malleus
    • Mandible
    • Maxilla
    • Zygomatic
    • Squamous part of temporal bone
• What are the derivatives of the second pharyngeal arch?
  • Nerve: facial nerve
  • Arteries
    • Hyoid and stapedial arteries
  • Muscles
    • Muscles of facial expression
    • Posterior belly of digastric
    • Stylohyoid
    • Stapedius
  • Bones and cartilages
    • Stapes
    • Styloid process of temporal bone
    • Part of hyoid bone
• What are the derivatives of the third pharyngeal arch?
  • Nerve: glossopharyngeal nerve
  • Arteries
    • Common carotid and part of internal carotid
  • Muscles
    • Stylopharyngeus muscles
  • Bones and cartilages
    • Part of the hyoid bone
• What are the derivatives of the fourth pharyngeal arch?
  • Nerve: superior laryngeal nerve
  • Arteries
    • Left fourth aortic arch: part of the aortic arch
    • Right fourth aortic arch: part of the right subclavian artery
  • Muscles
    • Cricothyroid
    • Levator veli palatini
    • Constrictor muscles of pharynx
  • Bones and cartilages
    • Forms the thyroid, and the cricoid, arytenoid, corniculate, and cuneiform cartilages of the larynx together with the sixth pharyngeal arch
• What are the derivatives of the sixth pharyngeal arch?
  • Nerve: recurrent laryngeal nerve
  • Arteries
    • Left sixth aortic arch: left pulmonary artery and ductus arteriosus
    • Right sixth aortic arch: right pulmonary artery
  • Muscles
    • Intrinsic muscles of larynx
  • Bones and cartilages
    • Forms the thyroid, and the cricoid, arytenoid, corniculate, and cuneiform cartilages of the larynx together with the fourth pharyngeal arch
• What are the first arch syndromes?
  • Syndromes characterised by abnormal migration of neural crest cells into the first two pharyngeal arches
  • Includes Treacher Collins syndrome and Pierre Robin sequence
  • Causes hypoplasia of the maxilla, mandible, etc.