Growth cone dynamics during the migration of an identified commissural growth cone.

We have used time-lapse video microscopy to study the behavior of a neuron, Q1, that pioneers the posterior commissure of the embryonic grasshopper. Our goal is to use time-lapse video as a tool to acquire a precise picture of normal development over time, and thereby identify stereotypic activities that might indicate important interactions necessary for proper formation of the commissure. We have identified specific and reproducible behaviors that suggest the presence of underlying cellular interactions that may play a role in pathfinding. In particular, the Q1 growth cone undergoes several morphological changes as it contacts the midline. As a commissural neuron, the midline may be a target in its outgrowth; Q1's typical response upon contacting the midline with its filopodia, however, is a rapid retraction. This inhibitory reaction can be overridden by contact with filopodia of its contralateral homolog. Q1's growth cone can translocate across the midline at an accelerated rate by a process resembling "filopodial dilation" (O'Connor et al., 1990) once the two Q1 growth cones meet. Ablation of the contralateral Q1 blocks Q1's advance across the midline. We have also analyzed in detail the behavior of individual filopodia to identify behavioral differences that could indicate differences in substrate adhesivity. Except for instances of filopodial dilation seen only at the midline, we found no significant asymmetries in rates of filopodial extension and retraction, or in the survival times of individual filopodia. We suggest that either the adhesive signal used by Q1 is relatively weak, requiring the integration of many adhesive interactions by many filopodia to be resolved, or the guidance cues may not be adhesive in nature.