KBP is required for axonal growth, cytoskeletal organization and myelination in the CNS. (A,B) TEM images of transverse sections through the ventral spinal cord of a wild type (A) and kbpst23 mutant (B) at 50 hpf. Axons with a diameter greater than 0.75 μm are indicated by a white asterisk. There are fewer large-diameter axons in the kbpst23 mutant. Cytoskeletal disorganization is apparent in the large Mauthner axon (M) in the kbpst23 mutant compared with wild type. Scale bars: 0.75 μm. (C,D) TEM images of transverse sections through the ventral spinal cord show high magnification views of the Mauthner axon (M) in a wild type (C) and kbpst23 mutant (D) at 50 hpf. There are more properly oriented microtubules (e.g. arrowheads) and neurofilaments (e.g. arrows) in the wild type than in the kbpst23 mutant (D) Mauthner axon. Scale bars: 200 nm. (E,F) TEM images of transverse sections through the ventral spinal cord show that there are more myelinated axons (white asterisks) in wild type (E) than in the kbpst23 mutant (F) at 72 hpf. Examples of large diameter kbpst23 mutant axons with a disorganized cytoskeleton are indicated by black asterisks. Scale bars: 0.75 μm. (G-I) TEM images of transverse sections through the ventral spinal cord show that there are more large-diameter myelinated axons in the wild type (G) compared with kbpst23 mutants (H,I) at 7 dpf. Scale bars: 2 μm.
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