Interactions at the Neural Crest ‐ Mesoderm Boundary

Instruction about craniofacial development typically highlights contributions of neural crest cells to pharyngeal arches, the face and skull. This approach emphasizes the uniqueness of the crest population, but obscures the dynamic and essential interactions between crest‐derived and mesodermal populations. An alternative approach is to focus on the properties that define these populations and facilitate communications at the boundaries between them. The use of transgenic mice with crest‐specific lineage markers has confirmed studies using avian embryos, and provided clarification where homologies were uncertain (e.g. frontal bone) or mammalian neomorphic structures present (e.g., thyroid cartilage). Blood vessels and skeletal muscles present a special challenge at the crest‐mesoderm boundary. Both have obligate mesodermal progenitors (e.g., angioblasts; myoblasts), and both receive contributions from crest‐derived mesenchyme (perivascular smooth muscle; fascia and tendons). Thus, both require lineage‐specific gate‐keeping at the boundary. Interactions that drive the 3‐dimensional morphogenesis and histogenesis of blood vessels and skeletal muscles are similar to those seen elsewhere in the embryo, e.g., the developing limb. This approach provides an opportunity to present craniofacial development in a manner that emphasizes unifying rather than regionally unique principles.