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Fig. 4 Sil is required within SCs for axonal wrapping. (A-C″) TEM of a cross-section of the PLLn at 3 dpf in control (A,A′), csp−/− (B) and csp−/− embryos injected with psox10-sil-P2A-mCherry (C-C″). Magenta asterisks indicate some large caliber myelinated axons (shown at higher magnification in A′, C′, C″). Scale bars: 0.5 μm (A-C); 0.2 µm (C′,C″). (D) Quantification of the percentage of myelinated axons relative to the total number of axons in control (average of 10.46±1.14%, nine nerves, n=5 embryos), csp−/− (0%, eight nerves, n=6 embryos) and csp−/− injected with psox10-sil-P2A-mCherry (average of 8.64±1.20%, three nerves, n=3 embryos) (****P≤0.0001; ns, P=0.4871). (E) Graph representing the distribution of axons relative to their diameter with 0.1 µm bin width at 3 dpf in controls, csp−/− and csp−/− embryos injected with psox10-sil-P2A-mCherry. (F-I″) Acetylated tubulin (Ac-tub) and Mbp immunolabeling in control (F-F″), csp−/− (G-G″) and csp−/−+psox10-sil-P2A-mCherry (H-I″) embryos at 3 dpf. Arrows indicate the PLLn in F and G, the myelin sheaths in F′, mCherry+ Schwann cells along the PLLn in H and I and the corresponding Mbp expression in H′ and I′. F″, G″, H″ and I″ are the corresponding merge images of F and F′, G and G′, H and H′, I and I′, respectively. Scale bars: 20 μm.