At this stage in culture, oligodendrocytes are actively myelinating large diameter axons. (A) An oligodendrocyte (white asterisk) extending a cellular process between small, unmyelinated axons to ensheath a large-diameter axon (arrowhead), while extending another process forming compact myelin around a different axon (black asterisk). (B) Most large diameter axons are in the process of becoming myelinated. (C) Ultrastructure of compact myelin seen in oblique long section. (D) Higher magnification of an axon undergoing early myelination; four complete wraps of membrane can be seen. (E) A detailed view of the myelin sheath reveals the interperiodal line (arrow) indicative of compact myelin. Scale bars = 1 μm in (A) and (D), 0.5 μm in (B), and 100 nm in (C) and (E).