The stem cells of the neural crest

In the vertebrate embryo, the neurectodermal neural crest cells (NCC) have remarkably broad potencies, giving rise, after a migratory phase, to neurons and glial cells in the peripheral nervous system, and to skin melanocytes, being all designated here as "neural" derivatives. NC-derived cells also include non-neural, "mesenchymal" cell types like chondrocytes and bone cells, myofibroblasts and adipocytes, which largely contribute to the head structures in amniotes. Similar to the blood cell system, the NC is therefore a valuable model to investigate the mechanisms of cell lineage diversification in vertebrates. Whether NCC are endowed with multiple differentiation potentials or if, conversely, they are a mosaic of different committed cells is an important ongoing issue to understand the ontogeny of NC derivatives in normal development and pathological conditions. Here we focus on recent findings that established the presence in the early migratory NC of the avian embryo, of a multipotent progenitor endowed with both mesenchymal and neural differentiation capacities. This "mesenchymal-neural" clonogenic cell lies upstream of all the other NC progenitors known so far and shows increased frequency when single cell cultures are treated with the Sonic Hedgehog signaling molecule. These findings are discussed in the context of the broad potentials of NC stem cells recently evidenced in certain adult mammalian tissues.