Figure 3: gpr56 mutant spinal cord axons are hypomyelinated.

(a) Schematic representation of a 5 dpf zebrafish. Larvae were cut between segments 5 and 6 (red dashed line) and prepared for TEM. Axis shows orientation of the embryo (D, dorsal; V, ventral; A, anterior; P, posterior). (b) Diagram of a 5 dpf zebrafish cross-section, dorsal is up (D), ventral is down (V). In this image, the spinal cord is in orange and includes neuronal cell bodies (blue) and myelinated axons (green). Ventral region used for quantification is boxed in green, dorsal region used for quantification boxed in magenta. Muscle in purple. (c–k) Representative TEM images from the ventral spinal cord of WT (c–e, N=6), gpr56^stl13/stl13 mutant larvae (f–h, N=5), and gpr56^stl14/stl14 mutant larvae (i–k, N=4) at 5 dpf. Higher magnifications of c,f,i are shown in d–e,g–h,j–k, respectively. (c,d,f,g,i,j) Myelinated axons are shaded green, unmyelinated large caliber axons (≥ 500 nm) are shaded orange. (e,h,k) Images from panels d,g,j without pseudocolour. (l) Quantification of the percent of myelinated axons in the ventral spinal cord of gpr56^stl13/stl13 (P<0.019) and gpr56^stl14/stl14 (P<0.039) compared with WT controls. (m) Quantification of the total number of axons in the ventral spinal cord of gpr56^stl13/stl13 (P<0.5) and gpr56^stl14/stl14 (P<0.78) compared with WT controls. (n,o) We also did not observe a significant difference in the number of myelin wraps per myelinated axon in mutants compared with control on the large caliber Mauthner axon (m) or on smaller caliber myelinated axons. (c,f,i) Scale bar, 1 μm. (d,e,g,h,j,k) Scale bar, 500 nm. (l–o) Quantification performed on a stereotyped 14 μm² region (b) in the ventral spinal cord. Student’s t-test used to test for statistical significance and error bars shown as±s.d. NS, not significant. Data represent two technical replicates.