After commissural axons originating in the dorsolateral spinal cord have crossed the midline, they turn into the longitudinal axis of the spinal cord, still in close contact with the floor plate.
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In a screen for guidance cues controlling axonal pathfinding at the floor plate, we identified a number of candidate genes. Using in ovo RNAi as a tool to induce gene silencing in a temporally and spatially controlled manner, we identified sonic hedgehog (SHH) as a guidance cue for postcommissural axons. During early stages of neural tube development, Shh acts as a morphogen. Because the floor plate is the source of Shh its concentration decreases with increasing distance from the floor plate. Depending on the concentration of Shh cells in the early neural tube adopt a specific cell fate. During later stages of spinal cord development, Shh was shown to attract commissural axons toward the midline. Finally, after commissural axons have crossed the floor plate they respond to Shh again, this time with a different receptor (Hedgehog interacting protein, Hip). The interaction of postcommissural axons with Shh results in their rostral turn along the longitudinal axis of the spinal cord.
|Bourikas, D., Pekarik, V., Baeriswyl, T., Grunditz, A., Sadhu, R., Nardó, M., and Stoeckli, E.T. (2005) Sonic hedgehog guides commissural axons along the longitudinal axis of the spinal cord Nat. Neurosci., 8: 297 - 304||pubmed|