MESODERMAL GUIDANCE OF PIONEER AXON GROWTH

Pioneer axons in insect legs are experimentally accessible model syste ms for the molecular identification and cellular localization of guida nce cues regulating the path of axon growth. A detailed study of the F e2 pioneer axons in the legs of the cockroach was performed to examine the diversity of guidance mechanisms. A detailed microscopic analysis of the axons at various points in their trajectory indicates that the Fe2 axons grow on a mesodermal substratum which contains the cues gui ding their growth along a stereotyped path. An identified pair of musc le pioneer cells (MPC) are likely to play an important role in enablin g the Fe2 growth cones to respond to mesodermal guidance cues. The add ition of heparan sulfate, heparitinase, and phosphatidylinositol-speci fic phospholipase C to the medium perturbs the in situ path of growth of the Fe2 axons and the location of the MPC in cultured embryos. This indicates a role for heparan sulfate proteoglycans and glycosylphosph atidylinositol-anchored proteins in axon guidance. When these results are compared to those of similar experiments performed on the well-cha racterized Ti1 axons, they indicate significant differences in the mec hanisms that are used for axon guidance. The Fe2 neurons are a good mo del for elucidating the mechanisms used to guide axon growth on nonmus cle mesodermal substrates often encountered in the periphery of verteb rate embryos. (C) 1997 Academic Press.