The relative roles of contact inhibition and contact guidance in orientation of axons extending on aligned collagen fibrils in vitro

Abstract

The orientation of axons, extending from ganglionic explants on a substratum of roughly aligned collagen fibrils, was studied by quantitative measurements, time lapse cinémicrography and scanning electron microscopy (SEM). Nerve growth factor was added at two concentrations for comparisons of dense and sparse outgrowth of axons. The radial orientation of axons on unoriented substrata, ascribed to contact inhibition, was modified by a tendency for the axons to follow the aligned collagen fibrils during extension by contact guidance. Deviation from the radial outgrowth was most pronounced in cultures with low densities of axons and was, furthermore, increased by a prolonged culture period. Contact guidance is suggested to be best described in statistical terms, viz. as an increased probability for the actively extending axon to follow the aligned fibrils. Contact guidance could thus be induced by probability differences in various directions for the establishment of adhesion points observed by SEM between the substratum fibrils and microspikes from axon growth cones. It is considered that the orientation patterns observed are created by an interaction between contact inhibition and contact guidance; the contact inhibition dominating orientation at high axon density, the contact guidance being gradually involved in orientation at lower axon densities. The results are suggested to be significant for extending axons and migrating cells on extracellular substrata in vivo.