Motor corticospinal axon turning at the pyramidal decussation requires Sema6A. (a-d) Sema6A mRNA expression along the ventral pyramidal tract and pyramidal decussation (black arrows) during early motor CST axon guidance. Red box in (a) indicates the region of Sema6A expression shown in sagittal views of E16 (b) and P0 (c) WT mice, which is absent in the sense control (d). (e, f) Unilateral BDA motor CST axon tracing of P20 Sema6A-/- mice. (e', f") Higher power views of arrowed areas in (e, f), respectively. A boxed area in (f) is enlarged in (f'). Very few axons cross at the pyramidal decussation in Sema6A-/- mice (black arrows in (e, e')). Instead, axons form aberrant tracts (red arrows in (e, f, f")) in the ventrolateral spinal cord (SpC). Note that the aberrant tract moves out laterally as it traces down to the ispilateral spinal cord. The slightly different locations of the ectopic ventrolateral tracts seen here as compared to those seen in the PLXA3/PLXA4-/- mice in Figure 3 are due to different rostrocaudal locations of the sections. Similar to that seen in PLXA3/PLXA4-/- mice, many of these ventrolateral axons branch back toward the contralateral dorsal cervical spinal cord, though they are mistargeted below the dorsal funiculus (green arrowheads in (f', f")). (g) Comparison of percentages of WT, PLXA3/PLXA4-/-, and Sema6A-/- mice with an abnormal ventral CST apparent with BDA tracing. Numbers in parentheses indicate the number of mice analyzed. (h) Average normalized areas (see Materials and methods) of CamKIIα-labeled dorsal CST axons in WT, PLXA3/PLXA4-/-, and Sema6A-/- mice. The dorsal CST area in each animal (mean ± standard error of the mean) is indicated by a black circle. The overall average dorsal CST area (black lines) is decreased in the cervical spinal cords of Sema6A-/- (n = 2 mice) versus WT (n = 6 mice) mice. **p < 0.01, Student's t-test. cc, central canal; DF, dorsal funiculus, IO, inferior olive. Scale bars: 500 μm (b-f); 100 μm (e'f"); 25 μm (f').