Review
- Critical time for craniofacial development is wk 3-4 (same as gastrulation)
- Neural crest cells ⇨ bones of face & anterior skull
- Mesoderm ⇨ posterior bones & base of skull
- Lateral Plate Mesoderm ⇨ laryngeal cartilage
- 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)
- somitomere = incompletely differentiated (in head)
- somite = completely differentiated (found in lower body)
- somitomeres form all voluntary muscles in head / neck; but scaffold (patterning) is by connective tissue elements
- directed by HOX genes ⇨ populate specific regions of head/neck
- MSX-2 genes: specific “suicide” signal to NCCs along rhombomeres 3 & 5
- produces gaps (brachial grooves)
- ensures non-mixing between populations of NCCs
- 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
- NCCs provide link between development of face & brain; key BUT also very sensitive to teratogens
- develop during 4th 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
- blood vessel development follows path of least resistance (thru mesenchyme) => many run through brachial arches during early embryogenesis