Neural Patterning: Spinal Cord Segmentation and Somite Patterning

Abstract

Segmentation is conspicuous in the regular periodic spacing of vertebrate spinal (motor and sensory) nerves. Segmented spinal cord motor neurons and interneurons may have evolved in early vertebrates alongside the segmented somites. Zebrafish spinal motor neurons are organized segmentally, probably in response to somite-derived signals. In birds and mammals, spinal nerve segmentation is generated by an anterior-posterior (rostral-caudal) somite polarity. Somite segmentation and polarization is initially established via cyclical Notch/Delta, Wnt and Fgf signalling. Posterior half-somite cells then express repellent proteins which force neural crest cells and peripheral motor and sensory axons to migrate in the anterior half-somites, ensuring a proper register between spinal nerves and the segmented vertebral column. Diffusible repellents from surrounding tissues also guide spinal axons in the dorsoventral axis (“surround-repulsion”), so repulsive forces guide spinal axons in three dimensions.