Figure 4.

Downregulation of Sfrp1 interfered with rostrocaudal axon guidance. A–D, Injection and electroporation of dsRNA derived from Sfrp1 (dsSfrp1; A) and Sfrp2 (dsSfrp2; B), but not Sfrp3 (dsSfrp3; C) or Sfrp4 (dsSfrp4; D), interfered with the correct rostral (R) turning of post-crossing axons along the contralateral floor-plate border. In the absence of Sfrp1, many axons failed to cross the floor plate (indicated by dashed lines) and mostly failed to turn into the longitudinal axis (A). Occasionally, caudal (Ca) turns were observed (A, open arrowheads). Injection sites were classified as exhibiting a strong phenotype when >50% of the axons stalled in the floor plate or at the floor-plate exit site, or when axons turning caudally were found. E, According to these criteria, strong phenotypes were observed, on average, at 27.5 ± 5.2% of the injection sites in embryos lacking Sfrp1 (n = 26 embryos) (supplemental Table S2, available at www.jneurosci.org as supplemental material). A weaker effect was found when Sfrp2 was downregulated (B, E), as a strong phenotype was only seen at 18.3 ± 6.7% of the injection sites (n = 17 embryos). The value for Sfrp2 was not significantly different (ns) from values for Sfrp3, Sfrp4, and controls. The rostral turning of post-crossing axons was not affected in the absence of either Sfrp3 (C, E) (supplemental Table S2, available at www.jneurosci.org as supplemental material) (10.4 ± 4.8% strong phenotype, n = 20 embryos) or Sfrp4 (D, E) (supplemental Table S2, available at www.jneurosci.org as supplemental material) (12.2 ± 3.0% strong phenotype, n = 19 embryos). These values were not different from control embryos (wild type, WT), where, on average, 11 ± 5% of the injection sites per embryo exhibited a strong phenotype (n = 19 embryos). For statistical analysis the two-tailed Student's t test was used. Values are given +SEM. *p < 0.05 for dsSfrp1 compared with control, dsSfrp3, and dsSfrp4. Rostral is to the top in A–D. Scale bar, 60 μm.