Axons navigate to their targets during brain development by interacting with gradients of guidance molecules. Exactly how they read these gradients is unclear. Now on , Friedrich Bonhoeffer, Martin Bastmeyer and co-workers propose that chick retinal axons stop growing in response to the combined effect of the total amount of ephrin5A that they have encountered during their outgrowth and its local concentration. The researchers investigated the response of chick retinal growth cones to different gradients and local concentrations of ephrin5A by using a new technique - microcontact printing - to prepare reproducible and quantifiable, discontinuous gradients of this guidance molecule on coverslips. They found that the growth cones stopped at distinct zones within these gradients but remained active rather than collapsing. The position at which they stopped depended on both the steepness of the gradient and the local concentration of substrate-bound ephrin. Whether a similar mechanism acts in vivo - where growth cones are reading several gradients simultaneously - remains to be investigated.
