The Neural Crest, a Fourth Germ Layer of the Vertebrate Embryo

The term “fourth germ layer” has been used several times in the embryological literature with respect to the neural crest (NC). This is related to its remarkable features: pluripotency, attested by the large variety of the cell types that it yields, and the migratory properties of its component cells responsible for their widespread distribution throughout the vertebrate body. The NC, which underpins the origin of most of the vertebrate head, is a vertebrate innovation and considered to have played a key role in chordate evolution. In this chapter, we review the characteristics that have made vertebrates distinct from their protochordate ancestors, and we report new findings on chordates evolution that prefigure the apparition of vertebrates: migratory cells that are possible forerunners of the NC in the urochordate Ciona intestinalis and the presence in the rostral end of cephalochordates and hemichordates of a genetic scaffold where landmarks of the vertebrate brain can be recognized. A new role has been found for the NC in vertebrate evolution: the regulation of the development of the fore- and midbrain, through the production of anti-Bmp factors (like Noggin, Gremlin) controlling the production of Fgf8, an essential player in brain development at the early stages of neurogenesis.