Fig. 1. Substrate-bound gradients fabricated by microcontact printing. (A) Schematic of a growth cone in a continuous gradient. (B,C) Growth cones in different discontinuous gradients fabricated by microcontact printing: (B) a steep gradient built by an array of stripes and (C) a shallow gradient built by dots. Antibody-stained ephrin is shown in red; phalloidin-stained actin in black. (D,E) The gradients shown in B and C in lower magnification extend over a distance of 250 µm. The relative slope 1 of the gradient in D is reduced to 1/3 in E by interrupting the ephrin stripes. (F) Averaged fluorescence intensities measured in a 20-µm-wide measuring field moved along the axis of the gradients in D (blue curve) and E (red curve). Scale bars: 15 µm in B,C; 50 µm in D,E.