Contact guidance of chick embryo neurons on single scratches in glass and on underlying aligned human skin fibroblasts.

The influence of substratum topography on the morphology and orientation of neurites of chick embryo neurons was studied. Two series of experiments are reported. One concerned the behaviour of growth cones when the axons become contact-guided by the surface texture. The second studied contact guidance of neurites extending on a compact layer of fixed aligned human skin fibroblasts (HSF). It was observed that when the growth cones of sensory neurons isolated from dorsal root ganglions encountered a single scratch in a glass surface (0.1-2 microm in depth and diameter) they turned and continued movement following the axis of the scratch. These neurons became contact-guided as a result of the sequence of events. The growth cone filopodia recognized the irregularity in the substratum surface, whereas the growth cone lamella stabilized contact with the scratch and moved forward along the scratch axis. Scanning electron microscope revealed that the single scratches 150 nm in width and ca. 100 nm deep growth cone filopodia less than 200 nm in diameter could detect and react by turning into them. These filopodia extensions followed the edge of scratches. However, phase contrast and Nomarski's differential interference contrast appeared insufficient for analysis of primary contact guidance of fine growth cone filopodia which themselves are often less than 200 nm. In neuron cultures on fixed aligned HSF, the neuron aggregates assumed spindle-like shapes, and sparsely seeded individual neurons extended axons along the long axes of the fibroblasts. The axons extended significantly further on the fixed underlying fibroblasts than on collagen-covered glass. In crowded cultures of neurons, the cells extended neurites ignoring both the surface anisotropy (the scratches) and the orientation of the aligned fibroblasts. Immunofluorescence staining of neurons with antibodies against neurofilaments made it possible to analyse their shape and orientation on the fibroblasts. Computer-assisted image analysis permitted the observed alignment of the neurites to be characterized quantitatively.