DaVinci medical academy

No 55/2, New Avadi Road, Alagappa Nagar, Kilpauk. Chennai

Call Now.

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 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
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