Motor corticospinal axon turning at the pyramidal decussation is independent of neuropilins. (a-f) Unilateral BDA motor CST axon tracing was performed in (a, b, e, f), and bilateral BDA motor CST axon tracing was performed in (c, d). (a'-f') Higher power views of arrowed areas in (a-f), respectively. The abnormal ventrolateral CST is not observed at the pyramidal decussation (a) and cervical spinal cord (SpC) (b) of P45 NPN-1sema-/- mice. However, crossing motor CST fibers are noticeably defasciculated at the pyramidal decussation (black arrows in (a, a')) and the defasciculated axons form ectopic tracts in the contralateral spinal cord (black arrows in (b, b')). Motor CST axons in P45 NPN-2-/- (c, d) and Sema3A-/- (e, f) mice travel normally at the pyramidal decussation (black arrows in (c, c', e, e')) and cervical spinal cord (black arrows in (d, d', f, f')). (g) Comparison of percentages of WT and mutant mice with an abnormal ventral CST apparent with BDA tracing. Numbers in parentheses indicate the number of mice analyzed. This result indicates that in contrast to PLXA3/PLXA4-/- mice, there is no ventral CST in NPN-1-/-, NPN-2-/-, or Sema3A-/- mice. (h) Average width of the pyramidal decussation (mean ± standard error of the mean) in WT, PLXA3/PLXA4-/-, NPN-1-/-, NPN-2-/-, and Sema3A-/- mice. As expected, the width of the pyramidal decussation in PLXA3/PLXA4-/- mice (n = 6 mice) was smaller than WT mice (n = 5 mice). **p < 0.01, Student's t-test. In addition, the width of the pyramidal decussation was larger in NPN-1-/- mice (n = 4 mice) than WT, suggesting that CST axons are defasciculated in NPN-1-/- mice as they cross at the pyramidal decussation. *p < 0.05, Student's t-test. The width of the pyramidal decussation in NPN-2-/- mice (n = 2 mice) and Sema3A-/- mice (n = 4 mice) was similar to WT. cc, central canal; DF, dorsal funiculus. Scale bars: 500 μm (a-f); 100 μm (a'-f').