Motor corticospinal axon pathfinding is abnormal in mice lacking PLXA3 and PLXA4. (a-b') Diagram and sagittal view of the brain showing motor CST axons bilaterally labeled with DiI traveling within the pyramidal decussation (Pyr Dec; white arrow in (b')) and dorsal spinal cord (dSpC; white arrows in (b)) of P3 WT mice. (c-d') Diagram and sagittal view of the brain showing motor CST axons bilaterally labeled with DiI traveling past the pyramidal decussation and into the dorsal (white arrows in (d)) and ventral spinal cord (vSpC; yellow arrows in (d')) of P3 PLXA3/PLXA4-/- mice. Note that the white dashed lines in (b-b', d-d') indicate meninges surrounding the dorsal and ventral edges of the spinal cord and do not represent positive DiI labeling. (e) Diagram showing bilaterally labeled CST axons in P30 PLXA3/PLXA4-/- mice. In all diagrams (a, c, e), the normal motor CST axonal projection is indicated in blue and the abnormal ventral CST projection is indicated in yellow. (f-f") Course of BDA-labeled motor CST axons along the pyramidal tract in the brainstem (black arrowheads) of P30 PLXA3/PLXA4-/- mice. BDA-labeled axons were observed in the dorsal (dSpC; red arrow) and ventral (vSpC; black arrow) spinal cord. Higher power views of arrowed areas are shown in the insets of (f', f"). (g, h) Electron micrographs illustrating examples of BDA-labeled motor CST axons in the dorsal (g) and the ventrolateral (h) aspect of the cervical spinal cord in P30 PLXA3/PLXA4-/- mice. All labeled axons are myelinated (red arrows). (i) Average perimeters (mean ± standard error of the mean) of BDA-labeled axons are similar within the dorsal CST of P30 WT mice (n = 36 sections from 2 mice) and the dorsal (n = 13 sections from 2 mice) and ventrolateral (n = 26 sections from 2 mice) CST of P30 PLXA3/PLXA4-/- mice (dKO). p > 0.05, ANOVA, Neuman-Keuls test. (j) Average densities of axons (mean ± standard error of the mean of axons per 100 μm2) are similar in the dorsal CST of P30 WT (n = 36 sections from 2 mice) and PLXA3/PLXA4-/- mice (dKO; n = 13 sections from 2 mice). p > 0.05, Student's t-test. Each data set was averaged from randomly selected CST areas on all the electron micrographs taken from two animals. Scale bars: 500 μm (b-b', d-d'); 1,000 μm (f); 200 μm (f', f"); 1 μm (g, h).