preventing RGCs from growing into layers beyond the optic fiber layer. Axons from the RGCs will grow and extend towards the optic disc, where they exit the eye. Once differentiated, they are bordered by an inhibitory peripheral region and a central attractive region, thus promoting extension of the axon towards the optic disc. CSPGs exist along the retinal neuroepithelium (surface over which the RGCs lie) in a peripheral high–central low gradient. Slit is also expressed in a similar pattern, secreted from the cells in the lens. Adhesion molecules, like N-CAM and L1, will promote growth centrally and will also help to properly fasciculate (bundle) the RGC axons together. Shh is expressed in a high central, low peripheral gradient, promoting central-projecting RGC axons extension via Patched-1, the principal receptor for Shh, mediated signaling. Growth into and through the optic nerve RGCs exit the retinal ganglion cell layer through the optic disc, which requires a 45° turn. This requires complex interactions with optic disc glial cells which will express local gradients of Netrin-1, a morphogen that will interact with the Deleted in Colorectal Cancer (DCC) receptor on growth cones of the RGC axon. This morphogen initially attracts RGC axons, but then, through an internal change in the growth cone of the RGC, netrin-1 becomes repulsive, pushing the axon away from the optic disc. This is mediated through a cAMP-dependent mechanism. Additionally, CSPGs and Eph–ephrin signaling may also be involved. RGCs will grow along glial cell end feet in the optic nerve. These glia will secrete repulsive semaphorin 5a and Slit in a surround fashion, covering the optic nerve which ensures that they remain in the optic nerve. Vax1, a transcription factor, is expressed by the ventral diencephalon and glial cells in the region where the chiasm is formed, and it may also be secreted to control chiasm formation. Growth at the optic chiasm When RGCs approach the optic chiasm, the point at which the two optic nerves meet, at the ventral diencephalon around embryonic days 10–11 in the mouse, they have to make the decision to cross to the contralateral optic tract or remain in the ipsilateral optic tract. In the mouse, about 5% of RGCs, mostly those coming from the ventral-temporal crescent (VTc) region of the retina, will remain ipsilateral, while the remaining 95% of RGCs will cross. This is largely controlled by the degree of binocular overlap between the two