Figure 8.

Serial section SEM reconstructions reveal that axons tunnel for long distances within basal skin cells at 78 hpf. A: 3-D reconstruction of 4.55 μm of serial ultrathin sections. The basal skin cell (blue) is only partially reconstructed to reveal axon structure. Two axons are enveloped within the basal cell (red and orange). The orange axon was present in all sections and varied in diameter from 0.4 to 2 μm before terminating in the final section. B: Same as in A, shown from an apical view. C: A single SEM section, from the location indicated with arrows in A. Red and orange arrows point to enveloped axon profiles of the red and orange axons reconstructed in A. D: Same orange axon as in A, made transparent to reveal the presence of reticular mitochondria inside the axon. Arrows indicate the location of the single SEM section shown in the inset. E: 3-D reconstruction of 1.68 μm of serial ultrathin sections. Two basal skin cells (yellow and blue) engulf two axons (purple and orange), which travel longitudinally through the basal cells. The orange axon crosses between the two basal skin cells while remaining continuously enveloped. Arrow indicates the location of the single SEM section shown in the inset. F: 3-D reconstruction of 1.61 μm of serial ultrathin sections. A single basal skin cell (blue) envelops two axons (purple and orange), which travel longitudinally through the basal cell. The orange axon has a large varicosity, shown in single SEM section in G. Orange and purple arrows point to membranes of the orange and purple axons reconstructed in F. H: Two serial SEM sections of the purple axon (arrow in F) show that the convoluted skin cell membrane channel that envelops the axon comes from different sides of the basal cell as the axon approaches the basal cell boundary. To view the reconstructions presented in these figures from multiple angles, movies showing 360° rotations of each reconstruction can be found at the website http://www.mcdb.ucla.edu/Research/Sagasti/3Drotations. Scale bars = 1 μm.