Development of a sensory afferent projection in the grasshopper embryo

The morphogenesis of several types of sensory axon branching patterns has been described by cobalt filling the cereal nerve of the grasshopper embryo at a series of different stages in development, thus staining the earliest sensory axons as they grow through the CNS. This embryonic sensory projection contains all five types of cereal afferents seen in the adult, and no new sensory tracts are added during postembryonic life. When the embryonic sensory axons first follow their pioneer axons into the neuropil they choose pathways which are characteristic of the adult sensory tracts. Since the afferents follow these paths without sending collaterals into the other tracts, it appears that the growing axon chooses its specific pathway without extensive exploration of alternative routes. Likewise, nearly all of the branches which arise from the embryonic sensory axons remain within the eventual domain characteristic of each cell type. This precise, determinate pattern of initial growth implies that the sensory axons are guided through the neuropil and achieve their final branching patterns with a minimum of overgrowth and pruning. The fact that initial growth is so precise also suggests that the parameters which guide the growing axon may help to determine its eventual pattern of synaptic connectivity by limiting its physical access to large portions of the neuropil which contain potentially compatible synaptic partner cells. Two different types of neurons may be supplying the sensory afferents with guidance cues: (i) Although most of the cereal sensory axons diverge from the cereal pioneer axons within the CNS, some sensory afferents continue to follow the pioneers through several ganglia, (ii) In the adult, a large number of the cereal sensory axons form a hollow shell of arborization around the main dendrite of an identified synaptic target cell, the Medial Giant Interneuron (MGI). This structure, the interneuron dendrite and the shell of sensory arbor, is called the cereal glomerulus. Since the MGI's dendrite is already present at the stage when the first sensory axons enter the CNS, interactions between these cells could serve to guide the glomerular sensory axons away from the pioneers into their future tracts.