We produced a monoclonal antibody, named A20, which specifically recognizes a 35 kDa protein and stains myelinated axons in
zebrafish brain. The A20 antigen is located at the outside of the myelin layer of large axons, and comprises a fine meshwork composed of
thin unit fibers about 1–2 μm in length and about 100–200 nm in thickness. The unit fibers form pentagonal and hexagonal structures,
which further polymerize into an envelope structure on the axons. The A20 monoclonal antibody did not stain neuronal cell bodies nor
synapses. Instead, the distribution of the A20 antigen was along axons, practically coincident with the distribution of myelin basic protein.
The monoclonal antibody stained only axons in the central nervous system (CNS), and not the extracellular matrix surrounding Schwann
cells. These results suggest that this antigenic meshwork (which we call the periaxonal net) is synthesized by oligodendrocytes. During the
development of the zebrafish brain, the periaxonal net appeared after the formation of myelin on the axons. The periaxonal net developed
first at the brain stem, then gradually appeared at the caudal end of the spinal cord. The thickness of the periaxonal net around the
Mauthner axon changed during development. Although the thickness of the Mauthner axon continues to grow throughout life, the
thickness of periaxonal net stopped growing at 6 months after fertilization
