New Observations on the Ultrastructure of the Membranes of Frog Peripheral Nerve Fibers

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Abstract
The surface membranes of axons and Schwann cells in electron micrographs each consist of pairs of dense lines about 25 A thick separated by light zones about 25 A thick making a unit about 75 A across. The principal 250-300 A double membranes (axon-Schwann membranes and mesaxons) of "C" fibers consist of two of these 75 A unit membrane structures in apposition separated by a gap approximately 150 A across. In some instances in myelinaled fibers, the unit membranes are in contact occluding the gap. Myelin is formed of a mesaxon with its central gap occluded in this way. This double membrane structure is wrapped spirally about the axon with its encircling inner (cyto-plasmic) surfaces in contact and zippered up into the compact myelin structure. The major dense lines repeating at a radial period of about 120 A are then formed by the contact of the inner surfaces of the 75 A unit membranes and the intraperiod lines by contact of the outside sur- faces. It is shown directly that the 75 A unit cell membrane structure contains half of the constituents present in each of the myelin repeating lamellae.