Upper Respiratory Tract Development

• Structures of the upper respiratory tract include the nose, pharynx, and larynx.

Nose Development

• Nasal placodes, which are bilateral oval thickenings of the surface ectoderm, appear on the inferolateral parts of the frontonasal prominence (FNP) by the end of the 4th week.
• Nasal placodes are the origin of the nasal epithelium.
• Initially convex, the placodes flatten into depressions.
• Mesenchyme proliferation around the placodes forms horseshoe-shaped medial and lateral nasal prominences.
• Nasal pits, which are depressions containing the nasal placodes, become the anterior nares (nostrils) and nasal cavities.
• Maxillary prominences enlarge due to mesenchyme proliferation and grow medially, causing the medial nasal prominences to move toward each other.
• The lateral nasal prominences are separated by the nasolacrimal groove.

Pharynx Development

• Pharyngeal Grooves:
  • The head and neck of the embryo develop four pharyngeal grooves on each side during the 4th and 5th weeks.
  • These grooves separate the pharyngeal arches externally.
  • Only the first pair of grooves persists postnatally, forming the external acoustic meatus (ear canals).
  • The other grooves form the cervical sinus, but are usually obliterated as the neck develops.
• Pharyngeal Membranes:
  • Pharyngeal membranes appear in the floors of the pharyngeal grooves where the epithelia of the grooves and pouches meet.
  • The endoderm of the pouches and the ectoderm of the grooves are separated by mesenchyme.
  • The first pharyngeal membrane, along with mesenchyme, becomes the tympanic membrane.

Larynx Development

• The epithelial lining of the larynx comes from the endoderm of the cranial end of the laryngotracheal tube. The cartilages of the larynx come from the 4th and 6th pairs of pharyngeal arches.
• The laryngeal cartilages develop from mesenchyme derived from neural crest cells.
• Mesenchyme proliferation at the cranial end of the laryngotracheal tube results in paired arytenoid swellings that grow toward the tongue.
• This changes the primordial glottis into a T-shaped laryngeal inlet, narrowing the developing laryngeal lumen.
• Rapid proliferation of the laryngeal epithelium temporarily blocks the laryngeal lumen, but recanalization occurs by the 10th week.
• Laryngeal ventricles, bounded by mucous membrane folds, form during recanalization. These folds become the vocal and vestibular folds.
• The epiglottis develops from the caudal part of the hypopharyngeal eminence.
• The hypopharyngeal eminence results from mesenchyme proliferation in the ventral ends of the 3rd and 4th pharyngeal arches.
• The rostral part of this eminence forms the posterior third of the tongue.
• Laryngeal muscles, originating from myoblasts in the 4th and 6th pharyngeal arches, are innervated by the laryngeal branches of the vagus nerves (CN X).
• The larynx and epiglottis grow quickly during the first 3 years after birth.
• The epiglottis attains its adult form by this time.