Head and Neck Development Quiz

Head and Neck Development

• The vertebrate cranial region consists of a neuro-cranium associated with the brain and major sense organs, and a viscera-cranium, formed from the pharyngeal (branchial) arches, associated with the oral region and pharynx.
• Embryonic components of the head and neck include the neural tube and ectodermal placodes, which form the brain and special sense organs.
• The neural crest contributes to the face, palate, tongue, pharynx, larynx, external and middle ear, and also forms intrinsic eye muscles.
• Cranial paraxial mesoderm forms the muscles of the head, including extrinsic eye muscles.
• Occipital somites form the occipital part of the skull and tongue muscles.

Pharyngeal Apparatus

• The pharyngeal apparatus consists of pharyngeal arches, pharyngeal pouches, and pharyngeal grooves or clefts.
• Pharyngeal arches (1, 2, 3, 4, and 6) are composed of mesoderm and neural crest cells, and develop during weeks 4-5.
• Each pharyngeal arch has its own artery, cranial nerve, muscles, and skeletal element.
• Pharyngeal pouches (1, 2, 3, 4) are lined with endoderm.
• Pharyngeal grooves or clefts (1, 2, 3, and 4) are lined with ectoderm.

Structures Derived from Pharyngeal Arches

• Structures derived from pharyngeal arches include the face, palate, tongue, thyroid gland, pharynx, larynx, and external and middle parts of the ear.

Structures Derived from Pharyngeal Pouches

• The 1st pharyngeal pouch gives rise to the tubotympanic recess, tympanic membrane, tympanic cavity, mastoid antrum, and pharyngotympanic tube.
• The 2nd pharyngeal pouch gives rise to the tonsillar sinus or fossa, and tonsillar crypts.
• The 3rd pharyngeal pouch gives rise to the thymus and inferior parathyroid glands.
• The 4th pharyngeal pouch gives rise to the superior parathyroid gland.

Structures Derived from Pharyngeal Clefts or Grooves

• The 1st pharyngeal cleft persists as the external acoustic meatus or ear canals.
• The other grooves lie in a slit-like depression – the cervical sinus – and are normally obliterated along with the sinus as the neck develops.

Development of the Face

• Mandibular processes fuse medially, forming the lower jaw.
• Medial and lateral nasal swellings grow and partially surround the paired nasal placodes, which deepen into two nasal cavities.
• These cavities remain separated by the nasal septum but are continuous with the oral cavity.
• Medial nasal swellings fuse together creating the intermaxillary segment, which forms the median part of the nose, philtrum, and the primary palate.
• Maxillary processes fuse with lateral nasal swellings (forming the side of the nose), and the nasolacrimal duct forms along this line of fusion.
• Maxillary and mandibular processes partially fuse forming cheeks.
• Inside the oral cavity, maxillary segments project downwards, either side of the tongue, as palatal shelves which elevate and fuse together in the midline, and also with the primary palate, creating the definitive palate.

Development of the Palate

• The palate develops from the primary and secondary palate.
• The primary palate develops from the innermost part of the intermaxillary segment.
• The secondary palate is formed of two shelf-like outgrowths from the maxillary swelling.
• These shelves fuse with each other to form the secondary palate during the seventh week.
• Anteriorly, these shelves fuse with the primary palate and cranially with the nasal septum.
• The incisive foramen is considered the midline landmark between the primary and secondary palate.
• The formation of the secondary palate causes separation of the primitive oral cavity into the definitive oral and nasal cavities.
• The definitive choanae are located at the junction of the nasal cavity and pharynx.
• If any of these fusions fail, congenital facial clefting occurs – most commonly cleft lip and palate.

Development of the Skull

• The base of the skull (chondro-cranium) is formed by the endochondral method of ossification.
• The skull vault (neuro-cranium) is formed by direct dermal ossification.
• Skull bones do not fuse together until early childhood, allowing the cranium to deform during birth then expand during childhood as the brain enlarges.

Development of the Thyroid Gland

• The thyroid gland does not develop from a pharyngeal pouch.
• It develops from the thyroid diverticulum, which forms from midline endoderm in the floor of the pharynx.
• The thyroid diverticulum migrates caudally to its adult anatomic position in the neck but remains connected to the foregut via the thyroglossal duct, which is later obliterated.
• The former site of the thyroglossal duct is indicated in the adult by the foramen cecum.

Tongue Development

• The copula is formed by the mesoderm of the second and cranial portion of the third pharyngeal arch.
• A large hypobranchial eminence grows rapidly and soon fuses with lateral lingual swellings along the line of demarcation, represented in the adult by the terminal sulcus of the tongue.
• The epiglottal swelling appears in the medial line and marks the development of the epiglottis.
• Immediately behind these swellings is the laryngeal orifice which is flanked by the arytenoid swellings.

Tongue Innervations

• The 1st and 2nd pharyngeal arches form the anterior 2/3 of the tongue (thus sensation via CN V3, and taste via CN VII).
• The 3rd and 4th pharyngeal arches form the posterior 1/3 of the tongue (thus sensation and taste mainly via CN IX, and extreme posterior via CN X).
• Motor innervation is via CN XII to hyoglossus (retracts and depresses the tongue), genioglossus (protrudes the tongue), and styloglossus (draws sides of the tongue upward to create a trough for swallowing).
• Motor innervation is via CN X to palatoglossus (elevates the posterior tongue during swallowing).
• Taste: CN VII, IX, X (solitary nucleus)
• Pain: CN V3, IX, X
• Motor: CN X, XII

Common Congenital Anomalies

• First arch syndrome results from abnormal formation of pharyngeal arch 1 due to faulty migration of neural crest cells, causing facial anomalies.
  • Treacher Collins syndrome and Pierre Robin sequence are two well-described syndromes involving neural crest cells.
• Pharyngeal fistula occurs when pouch 2 and groove 2 persist, forming a fistula generally found along the anterior border of the muscle.
• Pharyngeal cyst occurs when pharyngeal grooves that are normally obliterated persist, forming a cyst usually located at the angle of the mandible.

Ectopic Thyroid, Parathyroid, or Thymus

• Results from abnormal migration of these glands from their embryonic position to their adult anatomic position.
• Ectopic thyroid tissue is found along the midline of the neck.
• Ectopic parathyroid or thymus tissue is generally found along the lateral aspect of the neck.
• Thyroglossal duct cyst or fistula occurs when parts of the thyroglossal duct persist, generally in the midline near the hyoid bone.
  • The cyst may also be found at the base of the tongue (lingual cyst).
• DiGeorge sequence occurs when pharyngeal pouches 3 and 4 fail to differentiate into the parathyroid glands and thymus.
  • Neural crest cells are involved.

Other Congenital Anomalies

• Branchial fistula
• Tongue defects (aglossia, tongue tie)
• Facial clefts
• Cleft palate

Test your knowledge on the development of the head and neck in vertebrates. This quiz covers key concepts such as the neural crest's role, the pharyngeal apparatus, and the contributions of different embryonic components. Ideal for students of embryology and anatomy.