Fig. 8. Schematic diagram summarizing the function of dorsally derived netrin 1 on primary sensory afferents. (Left) In the E12.5 wild-type spinal cord, primary sensory axons have formed the dorsal funiculus in the dorsolateral margin of the spinal cord and extend longitudinally without extending collaterals into the dorsal spinal cord. At this stage, netrin 1 is expressed in the dorsal spinal cord and prevents the axons from entering the mantle layer. At E13.5, netrin 1 expression is decreased, allowing some collaterals (probably proprioceptive afferents) to enter the dorsal spinal cord. (Right) In the absence of netrin 1, the `waiting period' is abolished at this developmental stage (E12.5), and many sensory axons directly enter the dorsal mantle layer.