During the development of the retina, ganglion cell axons project towards the optic disc, a central opening in the retina through which the axons exit to form the optic nerve. But what guides the axons to the optic disc?According to Zhang and colleagues, the zinc-finger transcription factor Zic3 helps to pattern the chick retina for intraretinal axon guidance (see ). The researchers show that Zic3 expression is high at the periphery of the retina and low at the centre during active axon extension. Disruption of this gradient by retroviral overexpression of Zic3 results in aberrant axon projection but does not affect retinal neurogenesis or differentiation. Finally, the pattern of axonal growth over a `carpet' formed in vitro from alternating stripes of membrane fragments taken from the centre and the periphery of the retina suggests that Zic3 upregulates the expression of a negative axon guidance cue.
