Interference reflection microscopic study of dorsal root growth cones on different substrates: assessment of growth cone-substrate contacts

The contact between dorsal root growth cones and substrates of type IV collagen, fibronectin, poly-L-lysine, and laminin were studied using interference reflection microscopy (IRM) coupled with detaching growth cones from these substrates and observing substrate-associated membrane (SAM). IRM images of growth cones on collagen and fibronectin indicate substantial dark areas of close association between the substrate and basal region of the growth cone. The thin membranous veils and lamellapodia appear bright, indicating a greatly decreased association with the substrate. Filopodia can appear either entirely dark or light but usually are dark interspersed with brighter areas indicating varying degrees of contact. IRM images of growth cones on polylysine and laminin suggest a decreased association between the basal region of the growth cone and the substrate. The appearance of veils and lamellapodia was similar to that observed on collagen and fibronectin. Observations of SAM indicate an increased degree of contact between growth cones and substrates of collagen and fibronectin compared to laminin and polylysine. Collectively these data indicate that simple increases in adhesion are not adequate to explain substrate preference and enhancement of neurite elongation. Overall decreases in adhesion may accomplish the same phenomena, suggesting that it is perhaps more important to consider the regional distribution of adhesive contacts in relation to growth cone movement.