AXON TRACTS CORRELATE WITH NETRIN-1A EXPRESSION IN THE ZEBRAFISH EMBRYO

Netrins are secreted molecules that can attract or repel growth cones from a variety of organisms. In order to clarify the extent and scope of the effects of netrins for guiding growth cones, we have analyzed n etrin-1a within the relatively simple and well-characterized nervous s ystem of zebrafish embryos. netrin-1a is expressed in dynamic patterns that suggest that it guides the growth cones of a wide variety of neu rons. The spatiotemporal relationship of netrin-1a expression and exte nding growth cones further suggests that netrins may act to delineate specific pathways and stimulate axonal outgrowth in addition to attrac ting and repelling growth cones. Furthermore, aberrant outgrowth by co mmissural growth cones in the spinal cords of floating head mutants, i n which netrin-1a expression is altered, is consistent with an in vivo , chemoattractive action of netrin-1a. These data suggest that netrins act on many growth cones and influence their behavior in a variety of ways.