The neural crest

Abstract

It is generally accepted that the vertebrates evolved from protochordates ancestors, probably from an animal akin to modern day Amphioxus, a sessile filter feeder whose anatomical organisation is in many ways very similar to that of vertebrates. It has a dorsal nerve cord, a notochord, segmented muscle blocks and pharyngeal gill slits, but it lacks some key characteristics, which have their embryonic origin in the neural crest. These include the presence of sensory, sympathetic and parasympathetic peripheral ganglia, schwann cells, melanocytes, teeth and the skeletal elements of the head. Indeed, it has been suggested that the evolution of the neural crest was pivotal to the evolution of the vertebrates.Interestingly, recent studies have suggested that the genetic framework used for the generation of the neural crest in vertebrates was already in place early in chordate evolution. It has been found that in Amphioxus there is the Bmp2/4 gene, which is expressed in the non-neural ectoderm. Furthermore, orthologues of the msx and slug/snail genes have been identified in amphioxus and found to be expressed in the neural plate abutting the non-neural ectoderm. However, there is no direct evidence of neural crest cells in Amphioxus. Rather, it is likely that these genes are, as they are in vertebrates, involved more generally in the development of the dorsal nerve cord. These results would, however, be consistent with the neural crest evolving as part of an elaboration of the genetic programme, which patterns the dorsal neural tube. However, it is still as yet unclear how these dorsal neural tube cells first began to delaminate and to migrate into the periphery and thus to be neural crest cells.