GPR56 is required for efficient radial sorting. (A–F) TEM images of SN from (A and B) WT (n = 3), (C and D) Gpr56^+/− (n = 3), and (E and F) Gpr56^−/− mice (n = 4) on P3. SC precursor nuclei (N) surrounding and within axon bundles in Gpr56^−/− nerves. Higher magnification images: B, D, and F. Bars, 2 µm. (G) Quantification of axons sorted 1:1 with SCs (***, P < 0.0002, one-way ANOVA). (H) Quantification of the number of axons per unsorted bundle (*, P < 0.04, one-way ANOVA). (I–M) TEM images of P3 SN Gpr56^−/− (n = 4) animals. Bars: (I, J, L, and M) 1 µm; (I inset and K) 500 nm. (I) Large bundles of unsorted axons surrounded by SC processes are observed in a Gpr56^−/− SN. Some SCs also exhibited abnormal cytoplasmic protrusions (arrowhead). Higher magnification of the boxed in region shown within the inset. In addition, (J, L, and M) myelin abnormalities (outfoldings) and (K) nearly naked axons associated with SCs and partially surrounded by SC basal lamina (arrows) were often observed. (N) Quantification of the percent of axons with myelin abnormalities revealed a significant increase in myelin defects in Gpr56^−/− nerves compared with WT controls (**, P < 0.002, one-way ANOVA). (G, H, and N) Error bars represent SEM.