The neural crest and neural crest cells: discovery and significance for theories of embryonic organization.

The neural crest has long fascinated developmental biologists,and,increasingly over the past decades,evolutionary and evolutionary developmental biologists.The neural crest is the name given to the fold of ectoderm at the junction between neural and epidermal ectoderm in neurula-stage vertebrate embryos.In this sense,the neural crest is a morphological term akin to head fold or limb bud.This region of the dorsal neural tube consists of neural crest cells,a special population(s)of cell,that give rise to an astonishing number of cell types and to an equally astonishing number of tissues and organs.Neural crest cell contributions may be direct - providing cells - or indirect - providing a necessary, often inductive, environment in which other cells develop.The enormous range of cell types produced provides an important source of evidence of the neural crest as a germ layer, bringing the number of germ layers to four - ectoderm,endoderm,mesoderm,and neural crest. In this paper I provide a brief overview of the major phases of investigation into the neural crest and the major players involved,discuss how the origin of the neural crest relates to the origin of the nervous system in vertebrate embryos,discuss the impact on the germ-layer theory of the discovery of the neural crest and of secondary neurulation,and present evidence of the neural crest as the fourth germ layer.A companion paper (Hall, Evol. Biol.2008) deals with the evolutionary origins of the neural crest and neural crest cells.