Peripheral pathways are pioneered by an array of central and peripheral neurones in grasshopper embryos

Abstract

During embryonic development, neuronal growth cones traverse long distances along stereotyped pathways to arrive at their appropriate targets. Since Harrison's initial observations¹, increasing evidence has supported the notion that axonal pathways are “pioneered” early in development when the distances are short and the terrain relatively simple; thus an axonal scaffold is erected upon which later neurones can navigate. Grasshopper embryos have emerged as an attractive preparation in which to study pathfinding because the nervous system is relatively simple, and the individual identified neurones in the periphery and central nervous system (CNS) of the embryo are large and highly accessible. How are the first axonal pathways established between the periphery and CNS? Bate² has described a single pair of cells in the limb buds, and two pairs in the antennae, which were thought to establish the very first peripheral pathways over distances of several hundred micrometres from the periphery to CNS; he called these pairs of cells ‘pioneer neurones’. Keshishian³ confirmed this finding and also described⁴ a second pair of pioneers in the limb buds. We report here a further analysis of peripheral pathfinding in the grasshopper embryo using a monoclonal antibody, called I-5, which stains the cells responsible for establishing the first pathways^5,6. In contrast to the earlier reports, we find a large array of peripheral cells responsible for establishing the first pathways, with both pathfinders and landmark cells spaced at short intervals along the route. Furthermore, growth cones from central motoneurones make important contributions to the first peripheral pathways⁶.