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

Axon-Schwann membranes and mesaxons have hitherto been seen in electron micrographs as two single dense lines < 100 A wide separated by a light interzone about 150 A wide, the whole making a double membrane 250 to 300 A wide (4) (Fig. 7). I t will be shown in the present preliminary note that each of the supposedly single dense lines now appears as a pair of dense lines making a unit about 75 A in over-all thickness, and that this unit probably consists of a single bimolecular leaflet of lipide with associated protein. Fig. 1 shows a frog sciatic Remak fiber fixed in permanganate (7). Two axons (ax.) are seen and each is partially surrounded by a Schwann cell. All of the structures included between the arrows 1 make up the axon-Schwann membrane. These structures consist of two pairs of parallel dense lines separated from one another by a light space about 25 A wide. Each dense line is about 25 A thick so that each of the pairs makes a unit about 75 A across? The two units are separated from one another by a light space about 150 A wide. At arrow 2 a similar set of paired dense lines are seen separating the two Schwann cells. In Fig. 6 a mesaxon (m.) is seen to have the same structure. Text-fig. 1 is a semidiagrammatic tracing of Fig. 1. Measurements of the