Myelin formation in the peripheral nervous system. A: The Schwann cell has surrounded the axon, but the external surfaces of the plasma membrane have not yet fused in the mesaxon. B: The mesaxon has fused into a five-layered structure and spiraled once around the axon. C: A few layers of myelin have formed but are not compacted completely. Note the cytoplasm trapped in zones where the cytoplasmic membrane surfaces have not yet fused. D: Compact myelin showing only a few layers for the sake of clarity. Note that Schwann cell cytoplasm forms a ring both inside and outside of the sheath. (Modified from [4]. With permission of Lea & Febiger, publishers.)