Contact-mediated mechanisms of motor axon segmentation

In the chick embryo, the segmental pattern of motor outgrowth depends on guidance cues provided by sclerotome cells. Motor axons preferentially invade the anterior sclerotome but avoid the posterior sclerotome. To determine how motor growth cone motility is influenced by these cells, we used videomicroscopy to analyze the behavior of motor growth cones as they confronted identified sclerotome cells in vitro. After contact, motor growth cones invariably avoided posterior sclerotome cells by either branching or turning. Both types of avoidance behavior were initiated by a local inhibition of veil protrusion: veils failed to progress along the contacting filopodia. This inhibition was specific to veils since contact failed to alter the number of filopodia protruded. Moreover, motor growth cones turned away from posterior cells despite more persistent filopodial contacts with these cells than with the laminin substratum. In no case did contact with posterior cells cause a complete loss of growth cone motility or a complete collapse of growth cone structure. In contrast, motor growth cones exhibited a selective affinity for anterior cells, preferring the surfaces of these cells to the laminin substratum. Contact with anterior cells stimulated a generalized increase in protrusive activity: contact caused a net increase in the extension of veils and filopodia both locally and at sites distant from the site of contact. Contact also elicited a localized thickening of contacting processes, suggesting that contact with anterior cells promotes neurite consolidation. This behavior of motor growth cones in vitro suggests that both an inhibition of veil formation by posterior cells and an enhancement of motility and axon consolidation by anterior cells contribute to the preferential advance of motor axons into anterior sclerotome in vivo. We suggest that patterned outgrowth results from the juxtaposition of two contrasting environments that differentially influence growth cone motility.