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PRECAMBRIAN PERIOD

600-555 million years ago

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worm nervous system

THE NERVOUS SYSTEM

Ancestral coelomates evolved a number of fundamental features which would be retained in their vertebrate descendants such as a brain divided into 3 regions (forebrain, midbrain, and hindbrain with an important midbrain/hindbrain boundary) and homologous gene expression patterns.

Are the divisions of vertebrates and advanced invertebrates homologous? In other words, were some of the divisions of the brain of advanced vertebrates inherited from invertebrate ancestors? Molecular and developmental evidence supports the conclusion that the brains of bilateran animals are homologous organs which are derived from a common ancestral structure (Reichert, 2001). Both vertebrates and higher invertebrates (such as Drosophila) have the common feature of a brain which is divided into a forebrain, midbrain, and hindbrain with an important midbrain/hindbrain boundary characterized by the expression patterns of Otx, Hox, and Pax2/5/8 genes (Hirth, 2003). The brain of urochordates seems to be divided into three regions: a forebrain/midbrain (as indicated by the expression of Hroth, the homolog of vertebrate Otx), an anterior hindbrain (as indicated by the tunicate homolog of vertebrate Pax genes), and a posterior hindbrain (as indicated by the expression of a tunicate Hox gene) (Wada, 1998).
In coelomate animals, the gene Evx has also been recruited into the development of the nervous system. In vertebrates (but not Amphioxus), the role of Evx in the nervous system is augmented to include expression in the midbrain-hindbrain boundary (MHB) which is an important organizing region for the vertebrate brain. (Ferrier, 2001). Pax6 expression patterns help to mark the diencephalon-mesencephalon border in lampreys and gnathostomes (Derobert, 2002). The midbrain-hindbrain boundary is marked by the expression patterns of a number of additional genes such as Otx-2, Wnt-1, FGF8, En (2,5,8), and Pax(2,5,8) (Hall, 1999). Hox genes are involved in the segmentation of the hindbrain. For example, Hoxb-1 is produced only in rhombomere 4 while Hoxa3 is expressed at the boundary between rhombomeres 4 and 5 (Hall, 1999).