Cell-cell interactions during the migration of an identified commissural growth cone in the embryonic grasshopper.

One of the fascicles of the posterior commissure of the embryonic grasshopper is pioneered by an individually identifiable neuron named Q1. Q1 initially grows along a longitudinal pathway established by another pioneer neuron, MP1, and then crosses to the midline, where it meets and fasciculates with the axon of the contralateral Q1. The Q1 growth cone follows the contralateral Q1 axon to the contralateral longitudinal pathway, where it then fasciculates with axons of the MP1/dMP2 fascicle. In this work, we have identified a small set of early neurons that Q1 could use as guidance cues while negotiating its way along a specific and stereotyped pathway to the midline. Furthermore, we have observed characteristic morphological changes in the Q1 growth cone that could indicate responses to changing adhesivity in the substrates it contacts. We have also quantified the pattern of dye coupling between neurons in this system. Most of the neurons to which Q1 becomes coupled retain a strong, consistent pattern of dye coupling that shows no recognizable variation at times when growth cones are making pathway decisions. However, we have found one clear instance of transient, site-specific dye coupling between the Q1 growth cone and the ipsilateral MP1 soma. The timing and pattern of dye coupling in this system suggest that dye coupling may play a role in synchronizing the initiation of axon outgrowth among a small population of neurons. Although dye coupling may not play a direct role in neuronal pathfinding, it may exert a profound indirect influence on neuronal interactions by regulating the timing of axon outgrowth.