Fig. 9. Repulsive guidance cues that are likely to influence the contralateral commissural pathway. Several repellent guidance cues expressed in the spinal cord are ideally positioned to shape the contralateral trajectory of commissural axons. Slit proteins expressed by both floor plate cells and motoneurons may force newly crossed commissural axons to turn into the longitudinal axis (Li et al., 1999). A graded distribution of soluble repellent cues (e.g. one or more slits or semaphorins) secreted by floor plate cells or other ventral cell types may also act to deflect decussated commissural axons (shown in red) away from the ventral midline and into more dorsal spinal cord regions. In complementary fashion, secreted cues emanating from cells situated at or near the dorsal midline, including one or more slits, semaphorins and BMPs, may act coordinately with the local effects of B-class ephrins to specify the dorsoventral position of the longitudinal commissural tract. d, dorsal; fp, floor plate; rp, roof plate; v, ventral.