The motility of many cell types is controlled during development by Shh secreted by adjacent tissues (non-autonomous signalling). Now, unexpectedly,Cristina Sánchez-Camacho and Paola Bovolenta uncover a role for autonomous Shh signalling in the growth and guidance of mouse retinal ganglion cell (RGC) axons (see ). In mammals, the axons of contralateral RGCs (C-RGCs) cross the developing brain's midline (a source of Shh signals), whereas ipsilateral RGC(I-RGC) axons do not. The researchers first show that mouse C-RGCs but not I-RGCs express Shh. Then, by blocking Shh activity in vivo with antibodies,they show that midline-derived Shh funnels C-RGC axons to the contralateral side of the brain. Finally, by blocking Shh signal transduction in the RGCs themselves, they show that the outgrowth of C-RGCs is impaired well before they reach the midline, which indicates that the axons of these neurons require autonomously produced Shh for proper extension. Thus, the researchers conclude, Shh signalling influences growth cone behaviour both autonomously and non-autonomously.
