Growth cone navigation in substrate-bound ephrin gradients.

Graded distributions of ephrin ligands are involved in the formation of topographic maps. However, it is still poorly understood how growth cones read gradients of membrane-bound guidance molecules. We used microcontact printing to produce discontinuous gradients of substrate-bound ephrinA5. These consist of submicron-sized protein-covered spots, which vary with respect to their sizes and spacings. Growth cones of chick temporal retinal axons are able to integrate these discontinuous ephrin distributions and stop at a distinct zone in the gradient while still undergoing filopodial activity. The position of this stop zone depends on both the steepness of the gradient and on the amount of substrate-bound ephrin per unit surface area. Quantitative analysis of axon outgrowth shows that the stop reaction is controlled by a combination of the local ephrin concentration and the total amount of encountered ephrin, but cannot be attributed to one of these parameters alone.