Substrate pathways which guide growing axons in Xenopus embryos.

Optic axons from eyes transplanted to hindbrain and spinal cord (i.e., non-optic areas) in Xenopus embryos reproducibly follow a particular pair of continuous tracts, the external sensory tract and the internal sensory tract. The course of the external sensory tract may coincide with the course of the general somatic afferent tracts (i.e., the ascending and descending tracts of nerve V, the tract of Lissauer, and the Rohon-Beard cell tracts). The course of the internal sensory tract parallels that of the external sensory tract but runs more medially. Transplanted optic axons follow the same tracts regardless of where along the neural axis the axons have entered the CNS. What kind of developmental cues guide these optic axons? It is unlikely that the guidance cues are organized as spatial coordinates, rather it appears that the optic axons of these two tracts are guided along pre-existing substrate routes. We have called these routes substrate pathways. Substrate pathways may be a device normally used for organizing fiber tracts in the developing nervous system.