Head and Neck Development

Review

1. Critical time for craniofacial development is wk 3-4 (same as gastrulation)
2. Neural crest cells ⇨ bones of face & anterior skull
3. Mesoderm ⇨ posterior bones & base of skull
4. Lateral Plate Mesoderm ⇨ laryngeal cartilage

Somitomeres

1. Week 3: paraxial mesoderm differentiates into somitomeres
   • differentiate in cranial to caudal fashion
   • 7 somitomeres
   • key in segmenting brain into fore- mid- hind- brain segments by inducing neural tube (via chemogradients)
2. somitomere = incompletely differentiated (in head)
3. somite = completely differentiated (found in lower body)
4. somitomeres form all voluntary muscles in head / neck; but scaffold (patterning) is by connective tissue elements

Neural Crest Cells and Pharyngeal ArchesNCCs: migrate from neural folds in cranial-to-caudal sequence to somitomeres pharyngeal arches (brachial arches)

1. directed by HOX genes ⇨ populate specific regions of head/neck
2. MSX-2 genes: specific “suicide” signal to NCCs along rhombomeres 3 & 5
   • produces gaps (brachial grooves)
   • ensures non-mixing between populations of NCCs
3. Rhombomere: subdivision of hindbrain (segment of neural tube)
   • each rhombomere has its own HOX code
   • NCCs in each rhombomere migrate to brachial arches with compatible HOX code ⇨ specific link between face & brain development
4. NCCs provide link between development of face & brain; key BUT also very sensitive to teratogens

Pharyngeal Arches (Brachial Arches)General:

• develop during 4^th week of gestation
• lateral swellings on either side of head; result from local proliferation of NCC
• give rise to specific cartilages and bones
• pharyngeal grooves—external delineations between brachial arches; ectoderm covered
• pharyngeal pouches—internal deliniation between brachial arches; endoderm lined
• closing membranes—ectoderm/endoderm contact points ensuring non-mixing between brachial arches

Aortic Arch Arteries

1. blood vessel development follows path of least resistance (thru mesenchyme) => many run through brachial arches during early embryogenesis

Cranial Nerves—each is associated with a specific pharyngeal arch & innervates all its muscles, etcMuscles—each pharyngeal arch differentiates into a specific set of muscles which are innervated by the CN in that arch