The structure of the peripheral nerves of the lamprey (Lampetra fluviatilis)

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In the lamprey, the peripheral nerves are unmyelinated, but the axons are surrounded by Schwann cells. The larger axons have individual sheaths in which the number of Schwann cells visible in cross-sections varies with the size of the axons, so that while a fiber 5 μ in diameter has a sheath composed of between 4 and 6 Schwann cells, smaller fibers, about 0.3 μ in diameter, are enclosed by a single Schwann cell. Where adjacent Schwann cells come together in these sheaths, they are separated by a distance of 120–150 Å. A similar distance separates the axolemma from the inner surfaces of the Schwann cells, which have irregular outlines and frequently bulge into the axon, especially where they contain nuclei and mitochondria. The outer surface of the Schwann cells is more regular and is covered by a basement membrane. The structure of the sheaths is considered in relation to those of other animals.

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