PIONEER GROWTH CONE MIGRATION IN REGISTER WITH ORTHOGONAL EPITHELIAL DOMAINS IN THE GRASSHOPPER LIMB BUD

At the onset of neural development, pioneer growth cones can migrate o ver epithelia or neuroepithelia along stereotyped routes that establis h the pattern of initial neural tracts. These migration routes may ref lect the arrangement of distinct epithelial or neuroepithelial domains . In grasshopper limb buds, a pair of afferent pioneer neurons arise i n the tibia and their growth cones migrate on a stereotyped path throu gh the limb to the CNS. In the limb buds, circumferentially-oriented e pithelial domains expressing semaphorin-I, annulin, or alkaline-phosph atase, and a longitudinal domain, expressing engrailed, have been desc ribed. Using multiple-labeling techniques, we describe the relationshi ps of these domains to each other and to the pioneer neuron pathway. T aken together, these domains establish an orthogonal pattern of region ally specific epithelial molecular markers. During much of their migra tion across the limb epithelium, the pioneer growth cones are in regis ter with the axes of circumferential or longitudinal epithelial domain s.