embryology of oral cavity

• Embryology


  The study of the development of an organism from fertilization through birth
• Core knowledge


  Basic anatomy, Cell biology, Epithelium and connective tissues, Bone growth, Osteology, Genetics, Pedigrees, Mitosis & meiosis, Mutations and chromosomal aberrations, Developmental disease aetiology/epidemiology
• Three key phases


  • Phase 1: fertilisation to 3 weeks in utero
  • Phase 2: 4-8 weeks in utero
  • Phase 3: 9 weeks to birth*

  
• Rostral head fold


  Gives rise to all components of the head and face
• Rostral head fold formation


  1. Initially composed of frontal prominence, within which the anterior neural tube forms the brain; and six branchial (pharyngeal) arches (~day 25)

  2. Stomatodeum sits between frontal prominence and cardiac bulge, and is initially bounded laterally by the first pair of branchial arches
• Branchial arches


  • Form in pharyngeal wall from proliferating lateral plate mesoderm; subsequently reinforced by migrating NCCs
  • Are associated with formation of the face, palate, tongue, skull and dental arches
  • Externally separated by 'grooves', internally by 'pouches' – give rise to various structures
  • Consist of a cartilage rod (ectomesenchyme), muscular and vascular components (mesenchyme), and a neural component (neural crest)
  • First three are important in development of the face, mouth and tongue
• First branchial arch


  Mandibular arch gives rise to: Meckel's cartilage, muscles of mastication, maxillary artery and part of external carotid artery, 5th cranial nerve – trigeminal, maxillary process*
• Second branchial arch


  • Gives rise to: Reichert's cartilage, muscles of facial expression, 7th cranial nerve – facial
  • Third and fourth arches give rise to the 9th (glossopharyngeal) and 10th (vagus) cranial nerves
• Fusion


  • True fusion vs apparent fusion
  • What happens with 'fusion' of branchial arches compared with what happens with fusion of palatal processes via epithelial breakdown
• Formation of the face

  1. Nasal pits between lateral and medial nasal processes

  2. Fronto-nasal process

  3. Medial and fronto-nasal processes give rise to: middle portion of nose, middle portion of upper lip, anterior portion of maxilla, primary palate

  4. Maxillary processes of first branchial arch

  5. Odontogenic epithelium apparent as early as 27 days
• Formation of the palate


  1. Common oronasal cavity

  2. Primary palate

  3. Secondary palate: fusion of shelves from maxillary processes, directed downwards initially, tongue withdraws and shelves elevate, closure of secondary palate, loss of epithelium
• Formation of the tongue

  1. Local proliferation of mesenchyme in the floor of mouth

  2. Tuberculum impar plus paired lingual swellings from first branchial arch – mandibular nerve, give rise to mucous membrane of anterior 2/3

  3. Hypobranchial eminence from third branchial arch – glossopharyngeal nerve, gives rise to mucous membrane of posterior 1/3

  4. Muscles from occipital somites
• Development of the skull

  Cranial vault, Cranial base, Face, Chondrocranium, Endochondral ossification, Membrane bones
• Development of the mandible

  1. Meckel's cartilage acts as a scaffold, but mandible is membranous

  2. Ossification commences at division of IAN into mental and incisive branches

  3. Meckel's cartilage resorbed – persists as malleus

  4. Secondary cartilage: condylar (Week 12; persistent), coronoid, symphysial
• Development of the maxilla

  1. Ossification occurs in the maxillary process of the first branchial arch

  2. Ossification commences at division of maxillary nerve into anterior superior alveolar nerve and infra-orbital nerve
• Congenital defects


  • Genetic factors (e.g. Homeobox genes – transcriptional regulators)
  • Environmental factors (teratogens): infectious agents, ionizing radiation, drug exposure, hormones, nutritional deficiencies
  • Timing and duration of environmental exposure is critical (4-8 weeks, embryo is most susceptible)…for facial development, this may result in a combination of: molecular-level effects…cell signalling issues, cellular-level events…failure of NCC to migrate/differentiate, tissue-level events…failure of epithelium to degrade leading to fusion failure, organ-level events…failure of tongue to descend resulting in failure of palatal process elevation