Neuronal motility: the ultrastructure of veils and microspikes correlates with their motile activities.

We have documented the ultrastructural characteristics that correlate with protrusion, adhesion and retraction of neuronal veils and microspikes, by filming individual neurons of the chick ciliary ganglion and examining the same cells with high-voltage electron microscopy. We find that new veils invariably contain only a cortical meshwork of filaments and are devoid of microtubules, groups of vesicles and other organelles. At sites of recent veil retraction a cortical meshwork on the substratum side underlies a filament-free space containing vesicle clusters and a complexly folded upper membrane. Areas without veil activity are smooth-surfaced and contain a three-dimensional lattice of filaments. We discuss the implications of these observations for the mechanisms of surface recruitment and retrieval during motile activity. We also find that the ultrastructure of moving and attached extensions of the cell surface differs dramatically. Unattached microspikes and actively extending veils have an open, criss-cross array of filaments, whereas attached microspikes contain more aligned filaments, which extend as a small bundle into the growth cone. These results suggest that cell surface protrusion is mediated by meshworks of loosely packed filaments. More compact bundles of filaments are probably generated only at adhesion points.