Pathfinding by neuronal growth cones in grasshopper embryos. I. Divergent choices made by the growth cones of sibling neurons.

We are interested in how the growth cones of identified neurons navigate in the central nervous system of the grasshopper embryo. The behavior of identified growth cones was observed as a function of developmental time by (i) periodically removing embryos from synchronized clutches of eggs and (ii) filling identified neurons in the central nervous system with the fluorescent dye Lucifer Yellow. We examined the first six progeny of the identified neuronal precursor cell, neuroblast 7-4: the Q1, Q2, G, C, Q5, and Q6 neurons. Their growth cones extend sequentially upon the same route across the midline of the posterior commissure of the developing ganglionic neuropil. However, after reaching the contralateral side of the neuropil, their growth cones diverge from each other at reproducible, cell-specific choice points. By focusing our attention on one such choice point, at which the growth cone of the G neuron turns anteriorly while the growth cone of its sibling C neuron turns posteriorly, we hope to elucidate the environmental cues which guide individual growth cones.