Fig. 3

Nrg1 type III is required for myelin formation in the peripheral but not central nervous system. (A-C2) Transmission electron microscopy (TEM) images of transverse sections through the posterior lateral line nerve (PLLn) of zebrafish larvae of the indicated genotypes. (A,A2) At 5 dpf, nrg1^z26/+ nerves are beneath the epidermal basement membrane (black arrowheads) and Schwann cells (SC) have begun to myelinate axons (asterisks). (B,B2) nrg1^z26 mutant nerves that lack Schwann cells fail to transition across the basement membrane (black arrowheads) and remain in the epidermis where they are unmyelinated and defasciculated (open arrowheads). (C,C2) nrg1^z26 mutant nerves with ‘escaper’ Schwann cells are correctly positioned beneath the epidermal basement membrane (black arrowheads), but Schwann cell processes (arrows) fail to myelinate axons within the bundle. (A2,B2,C2) Higher magnification of the boxed regions in A,B,C. (D,E) TEM of transverse sections through the ventral spinal cord of nrg1^z26/+ (D) and nrg1^z26 mutant (E) siblings. Insets show myelin around the Mauthner axon. All images were taken at the level of the fifth somite. Scale bars: 2 μm in A,B,C; 1 μm in A2,B2 0.5 μm in C2 1 μm in D,E; 0.5 μm in D,E insets.