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.