Central myelin in the shark Scyllium stellare (Elasmobranchii, Selachii).

The central myelin of the phylogenetically ancient shark Scyllium stellare (Elasmobranchii, Selachii) was examined at the electron microscope. The known different chemical compositions of the myelin in higher and lower vertebrates is not associated in the Scyllium with a lack of any basic structural myelin elements, but is probably linked to variations in the myelin compactness and periodicity. Several unusual myelin structures and irregular patterns of unknown origin have been observed. The relationship between myelin-forming cells and the most external layers of myelinated structures suggest the possibility that the cytoplasmic glial wrapping is not the only mechanism responsible for central myelination in this vertebrate.