Segmentation: Spinal Cord Segmentation and A-P Somite Patterning

Segmentation is conspicuous in the regular periodic spacing of vertebrate spinal nerves. Segmented spinal cord motor neurons and interneurons may have evolved in early vertebrates alongside the segmented somites. Zebra fish 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 somite polarity. Somite segmentation and polarization is established via cyclical Notch/Delta, Wnt, and Fgf signaling, and posterior half-somite cells express contact repellents. These force axons and neural crest cells 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.