The First Growth Cones in the Central Nervous System of the Grasshopper Embryo

Abstract

What factors guide the advancing tips of individual growth cones during embryonic development? One way to answer this question is to examine and manipulate a developing embryo in which the growth cones of different neurons are confronted with the same environment and yet make different and stereotyped choices of which way to grow. The best such divergent choice to study would be one in which the individual growth cones are identifiable and highly accessible and their environment is stereotyped and relatively simple. We have been studying several different examples in the grasshopper embryo that meet these criteria: identified neurons with growth cones that make divergent choices (e.g., Goodman et al., 1982;Taghert et al., 1982; Raper et al., 1983a,b). In the example described in this chapter, the growth cones make divergent choices very early in embryogenesis when the terrain is relatively simple, the distances short, and the number of possible cells involved small. We will discuss the very first growth cones and the very first axonal pathways in the central nervous system (CNS) of the grasshopper embryo.