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Zebrafish as a model to study axon diameter. A Electron micrograph of a cross-section of the zebrafish ventral spinal cord at 5 dpf showing the diverse range of axon diameters. Five axons spanning the range of diameters are highlighted: the largest myelinated Mauthner axon (pink), two other myelinated axons (orange and green) and two unmyelinated axons (yellow and blue). B Schematic dorsal view of the larval zebrafish head with inset indicating the position of Mauthner neurons shown in right, labelled using the transgenic line Tg(hspGFF62A:Gal4); Tg(UAS:mem-Scarlet). The arrow points to one of the two axons, about to cross the midline. The image was obtained at 5 dpf. C Super-resolution confocal live-imaging time-course depicting the growth in diameter of a Mauthner axon (magenta—Tg(hspGFF62A:Gal4); Tg(UAS:mRFP)) with myelination (green—Tg(mbp:eGFP-CAAX)) at somite 15 from 2 to 5 dpf. D Tiled dorsal view of the Mauthner neurons (5 dpf), labelled using the transgenic line Tg(hspGFF62A:Gal4); Tg(UAS:mRFP). White boxed region depicts where the time course analysis in C was performed (somite 15). E Quantification of Mauthner axon diameter growth with relation to its myelination followed for the same axons over time at somite 15 from 2 to 5 dpf (n = 19 axons from individual animals). This dataset is the same as the wild-type data shown in Fig. 3B. F Schematic of the myelination of six different wild-type Mauthner axons at 3.5 dpf, with myelinated regions represented by black and unmyelinated regions represented by magenta, which is quantified in (G). Insets show a myelinated and unmyelinated region of the axon with the axon labelled using the transgenic lines Tg(hspGFF62A:Gal4); Tg(UAS:mRFP) and myelin labelled using the transgenic line Tg(mbp:eGFP-CAAX). G Quantification of the percentage of the Mauthner axon myelinated at 3.5 dpf and 5 dpf (n = 6 axons from individual animals). All graphs are presented as mean values ± SD, with repeated measures for the same axon at each time point. Scale bars: 1 µm (A), 10 µm (B, C, F), 100 µm (D). Source data are provided as a Source Data file.
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