Downregulation of PlexinA1 results in the same phenotype as seen in the absence of Sema6A. (a, b) Motoneurons streaming out of the ventral spinal cord identified by Isl-1 staining were only found after downregulation of PlexinA1 (arrows). The open arrow points to a motoneuron that is located in the ventral funiculus. Note that sensory neurons in the DRG (asterisk) are also stained by Isl-1. (e) Lack of none of the other PlexinAs enhanced the number of motoneurons found along the ventral roots compared to control-treated embryos (p = 0.0001 for the comparison between dsPA1 and all other treatment groups (indicated by three asterisks); values are given as mean ± standard error of the mean; see Figure 2b). (c, d) The phenotype seen after downregulation of Sema6A in dorsal BCCs was mimicked by both lack of PlexinA1 (c) and PlexinA4 (d). The effects of PlexinA downregulation were qualitatively different, however. In the absence of PlexinAs, the arrangement of DRGs, and not only the arrangement of their roots, was disorganized. (f) A phenotype was seen in 83% of embryos lacking PlexinA1 and in 67% of the embryos lacking PlexinA4. Bar 50 μm in (a, b); 200 μm in (c, d).