Embryology: Development of the Pharyngeal Apparatus

Understand the pharyngeal apparatus

  • Transient embryonic structure
  • Make important contributions to head and neck
  • Each arch has its own blood vessel, mesenchyme, outer ectodermal cover and inner endoderm (foregut) lining (show in the first picture below)
  • Between arches are pharyngealclefts (ectodermal side) and pharyngealpouches (endodermal side)

  • Class notes: The 1st pharyngeal arch is most cranial, 6th is most caudal. The first arch bifurcates into a maxillary and mandibular portion (show in the third embryo drawing C). the 2nd, 3rd, and 4th arches can also be seen in C. The pharyngeal clefts are the indentations in between the arches. The 1st cleft is between the 1st and 2nd arch, the 2nd cleft is between the 2nd and 3rd, and so on. [Note that there are only 5 arches labeled 1, 2, 3, 4, and 6. Dr. Johnson explains 4 and 6 as one, 4 being the cranial portion of 4 and 6 as the caudal part of 4.]

Understand role of neural crest in craniofacial development

  • Mesenchyme both in situ and from immigrant neural crest cells
  • Neural crest thought to populate ultimobranchial bodies and become C-cells of thyroid and may have role on thymic differentiation [class notes: the neural crest cells migrate to the 4th pharyngeal pouch and help make calcitonin secreting C-cells in thymus]
  • Brain neural crest cells contribute to pharyngeal arch mesenchyme, sensory placodes (e.g. otic placode), and cranial ganglia

Class Notes: Notice how portions of the foregut, midgut, and hindgut assist in the formation of pharyngeal arches

Know fates of arches, pouches, and clefts

  • First Arch
  • First cleft > external auditory meatus (remember that the pinna is formed from 6 auricular hillocks)
  • First pouch > auditory tube
  • Forms a Caudal mandibular process and Cranial maxillary process(there is bilateral fusion at the midline of the face forming the philtrum, incisors and part of the palate – this is a common place for congenital birth defects)
  • Meckel cartilage (hyaline cartilage where the mandible eventually forms, it eventually disappears – this has nothing to do with the actual formation of the mandible, which is formed from intramembranous ossification, as are most of the face bones)
  • malleus and incus (formed by endocondilar ossification, in which cartilage acts as the precursor of the bone)
  • CN V-Trigeminal Nerve
  • Muscles of mastication
  • Second Arch
  • Second cleft > obliterated
  • Second pouch > palatine tonsil
  • Stapes, styloid process, stylohyoid ligament, superior hyoid bone (the upper and lesser wing of hyoid bone)
  • CV VII-Facial nerve
  • Muscles of facial expression (starts at 2nd arch and migrates out toward the front of face)
  • Third Arch
  • Third cleft > quickly obliterated
  • Third pouch > inferior parathyroid and thymus glands
  • Inferior hyoid bone (lower and great wing)
  • CN IX-Glossopharyngeal Nerve
  • Stylopharyngeus muscle
  • Fourth and Sixth Arch (cranial and caudal portions of the forth arch, respectively)
  • Fourth cleft > quickly obliterated
  • Fourth pouch > superior parathyroid and ultimobranchial body > C-cells [
  • Laryngeal cartilages: thyroid, cricoid, arytenoid, corniculate, cuneiform
  • CN X-laryngeal branch of Vagus Nerve
  • Cricothyroid m., levator palatini, constrictors of pharynx

Cervical cysts

  • Second Arch Covers Caudal Clefts
  • Anomalies can create cervical cysts and/or fistulas
  • Lump or hole in neck anterior to sternocleidomastoid
  • Fistula would leak mucus from pharynx onto neck

Mandibulofacial dysostosis (aka First Arch Syndrome or Treacher Collins syndrome)

  • Autosomal dominant, point mutation, inheritance with variable penetrance (extreme cases exhibit hypoplasia, in which there is underdevelopment of the mandible)
  • Facial abnormalities
  • Deafness due to malformation of ossicles (malleus and incus, specifically)
  • Normal intelligence but can be mistaken for mental retardation because of deafness

DiGeorge Syndrome

  • Complex syndrome involving abnormal development of pharyngeal apparatus
  • Craniofacial anomalies
  • Thymic aplasia > opportunistic infections due to depressed T-cell function
  • Hypoparathyroidism
  • Cardiovascular defects
  • Probably caused by abnormal neural crest cell migration

Overview of Neural Crest Congenital Birth Defects

  • Class notes (for picture on left):
  • (A) example of 1st arch syndrome. The anterior portion of the ear has not developed. mandibular and maxiallary abnormalities.
  • (B) Robins sequence – extreme mandibular hyperplasia
  • (C) digeorges syndrome – subtle abnormalities in face, but leads to: hypertellorism, midfacial depression, repeat infections from opportunistic organisms/fungi bc of the Tcell disfunction, irregular thymus development, hypoplastic parathyroid (which leads to irregular calcium levels), and cardiovascular diseases
  • (D) hemifacial microsomia – abnormal micration of neural crest cells