part of myelin sheath compact myelin The portion of the myelin sheath in which layers of cell membrane are tightly juxtaposed, completely excluding cytoplasm. The juxtaposed cytoplasmic surfaces form the major dense line, while the juxtaposed extracellular surfaces form the interperiod line visible in electron micrographs. oligodendrocyte compact myelin cellular_component GO:0043218 NIF_Subcellular:sao1123256993 Schwann cell compact myelin cellular anatomical structure