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CAMBRIAN PERIOD

542-488 million years ago

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ganglion in mammal embryo

Urochordates lack the complex nervous system found in vertebrates and lack the specialized neural crest cells which form many of the uniques structures of the vertebrate nervous system and head (e.g. the ganglia of the mammal embryo pictured above). Modern tunicates do seem to possess a "proto-neural crest" given that many of the genes which are required by vertebrate neural crest cells are expressed along the urochordate neural tube.

Tunicates possess genes involved in the induction of neural tissue and for neural function which link them to vertebrates rather than protostomes (such as a greater diversity of Bmp and Wnt signals, the gene Nodal, SCO-spondin, noelin, and rhodopsin photoreceptors related to deep brain/pineal opsins of vertebrates) (Dehal, 2002). The genes required to form the neural crest did not appear suddenly in vertebrates. Neural crest induction involves genes such as BMP, chordin, FGF, TGF (at Hensen's node), WNT, Shh, dorsalin, odd-paired (a Drosophila pair rule gene), and the snail zinc finger gene. Many of these genes perform other functions as well and are known (or, at least, members of their gene families are known), in more primitive animals which lack neural crest cells.

A unique group of important embryonic cells called neural crest cells were a vertebrate innovation. Many of the characteristics which set vertebrates apart from more primitive chordates are determined by these neural crest cells. Neural crest cells develop into a number of cell types including sensory neurons, adrenergic neurons, cholinergic neurons, Rohon-Beard cells, satellite cells, and glial cells. As a result, the neural crest contributes to the brain, spinal ganglia, and the sympathetic and parasympathetic divisions of the ANS, among other structures (Hall, 1999). In tunicates, there is evidence of a protoneural crest in the pigmented cells along the neural tube (Hall, 1999). Although there are no neural crest cells in tunicates, the expression of the snail gene family member, Hrsna, indicates that some of the characteristics of neural crest cells may be present (Meinertzhagen, 2001). While Amphioxus lacks neural crest cells, it expresses many of the important genes involved in the differentiation of vertebrate neural crest cells in the region of the junction between the neural plate and the non-neural ectoderm. Thus, it appears that vertebrate neural crest cells employed genes which were already present in vertebrate ancestors (Ahlberg, 20-5).