Figure 7.

Model of Nrp2 function in mediating axonal guidance of dorsal commissural neurons in the developing spinal cord. A schematic diagram based on the in vivo and in vitro results illustrates Nrp2 expression observed as early as E9.5–E10.5 in the floor plate, when only few commissural neurons have begun to extend their axons. As precrossing axons approach the midline, from E10.5 to E11.5, floor plate expression of Nrp2 reaches its peak, resulting in the sequestering of Sema3B and thus allowing Nrp2/PlxnA1-positive axons to cross. When most of the commissural axons have crossed the midline, at E12.5–E13.5, floor plate Nrp2 is dramatically down-regulated, releasing Sema3B and promoting the exit of Nrp2/PlxnA1-positive axons away from the midline. From E13.5 on, the localization of axonal Nrp2 to the post-crossing segments of commissural axons coupled with the near absence of floor plate-derived Nrp2 provides guidance along the longitudinal axis toward brain targets while preventing the recrossing of the midline.