Spinal nerve segmentation in higher vertebrates: axon guidance by repulsion and attraction

The development of spinal nerve segmentation in higher vertebrate embryos provides a convenient experimental system for the analysis of axon guidance mechanisms. We review evidence from chick embryo experiments that segmentation of motor and sensory axons results from a combination of contact repulsion of axon growth cones by posterior somite cells and chemoattraction of growth cones by anterior cells. We also review progress in identifying the underlying molecular mechanisms in this system, and suggest a prominent role for carbohydrate groups in mediating growth cone repulsion.