THE BEHAVIOR OF OPTIC AXONS ON SUBSTRATE GRADIENTS OF RETINAL BASAL LAMINA PROTEINS AND MEROSIN

To study the behavior of optic axons to continuously changing concentr ations of their substrate, explants from embryonic retina were placed across gradients of retinal basal lamina proteins and merosin. The fol lowing growth patterns of axons in response to the substrate gradients were found: (1) Axons that grew up gradients, i.e., from low to high substrate concentrations, became longer and less fasciculated with inc reasing concentration of the substrate. On shallow basal lamina gradie nts, the axons also showed a directional response that resulted in gui dance to higher substrate concentrations. (2) Axons that grew down gra dients, i.e., from high to low substrate concentrations, became shorte r and more fasciculated with decreasing concentrations of the substrat e. On gradients of merosin, a significant alteration in the axonal gro wth direction toward higher substrate concentrations was detected. Axo ns heading down gradients never U turned to higher substrate concentra tions. (3) Axons confronted with discontinuous substrates were confine d to the borders of the substrate exclusively, whereas axons confronte d with substrate gradients were able to cross into the territory beyon d the substrate. (4) The growth patterns of axons on substrate gradien ts of basal lamina proteins and merosin were similar but not identical , indicating that axons may respond to substrate gradients dependent o n its chemical composition. The present results show that substrate gr adients can regulate length and fasciculation of neurites and have a l imited capability to direct axons to higher substrate concentrations.