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

Abstract

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.

The grasshopper median neuroblast is a multipotent progenitor cell that generates glia and neurons in distinct temporal phases

“…F, By 33% of development, one of the Q 1 pioneer growth cones (Myers and Bastiani, 1993b) has arrived at the midline (arrow), and a pioneer MFT axon has fasciculated with the Q 1 axon. Three irregular engrailed-expressing cells are indicated by arrowheads.…”

“…Grasshopper embryos were staged according to Bentley et al (1979). Embryo cultures were set up and maintained essentially as described by Myers and Bastiani (1993b). Under these conditions, embryos develop in a normal manner from 32-35% to over 45% in about 72 hr at 33°C.…”

Axon guidance at the midline choice point

“…Taken together with the in ovo results, these findings suggest that positive interactions between axonin-1 and Nr-CAM not only promote the entry of commissural growth cones/axons into the floor plate, but also render these axons insensitive to a collapse-inducing activity (CIA) that is unmasked by antibody perturbation . It is important to note in this regard that a midline-associated inhibitory activity for commissural growth cones has previously been identified in grasshopper embryos (Myers and Bastiani, 1993), and that floor plate-derived B-class ephrins have been shown to promote the collapse of commissural growth cones in vitro (Imondi et al, 2000). Furthermore, Slit functions as a midline repellent for commissural axons in the Drosophila ventral nerve cord (see Battye et al, 1999;Kidd et al, 1999 and below) and, possibly, in the vertebrate spinal cord (see Zou et al, 2000).…”

Axon fasciculation and differences in midline kinetics between pioneer and follower axons within commissural fascicles

“…Growth cones of pioneer and later axons differ in their morphology: early axons posses larger and more complex growth cones (LoPresti et al, 1973;Bastiani et al, 1984). And growth cone morphology changes correlate with specific choice points along the axon route (Myers and Bastiani, 1993).…”