Moore, A.C., Mark, T.E., Hogan, A.K., Topczewski, J., and Leclair, E.E. (2012) Peripheral axons of the adult zebrafish maxillary barbel extensively remyelinate during sensory appendage regeneration. The Journal of comparative neurology. 520(18):4184-4203.
Myelination is a cellular adaptation allowing rapid conduction along axons. We have investigated peripheral fibers of the zebrafish maxillary barbel (ZMB), an optically clear sensory appendage. Each barbel carries taste buds, solitary chemosensory cells, and epithelial nerve endings, all of which regenerate after amputation (LeClair & Topczewski, 2010). The ZMB contains axons from the facial nerve; however, myelination within the barbel itself has not been established. Transcripts of myelin basic protein (mbp) are expressed in normal and regenerating adult barbels, indicating activity in both maintenance and repair. Myelin was confirmed in situ using Toluidine Blue, an anti-MBP antibody, and transmission electron microscopy (TEM). The adult ZMB contains <180 small-diameter axons (< 2 μm), approximately 60% of which are myelinated. Developmental myelination was observed by whole-mount immunohistochemistry 4-6 weeks post-fertilization, showing myelin sheaths lagging behind growing axons. Early regenerating axons (10 days post-surgery), having no or few myelin layers, were disorganized within a fibroblast-rich collagenous scar. 28 days post-surgery, barbel axons had grown out several millimeters, and were organized with compact myelin sheaths. Fiber type and axon areas were similar between normal and regenerated tissue; within 4 weeks, regenerating axons restore <85% of normal myelin thickness. Regenerating barbels express multiple pro-myelinating transcription factors (sox10, oct6 = pou3f1 and krox20a/b = egr2a/b) typical of Schwann cells. These observations extend our understanding of the zebrafish PNS within the context of a little studied sensory appendage. The accessible ZMB provides a novel context for studying axon regeneration, Schwann cell migration, and re-myelination in a model vertebrate.