Developing Myelin in Human Peripheral Nerve

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Abstract
The process of myelination is examined in the axillary part of the radial nerve in human foetuses of 50 to 150 mm. At first bundles of unmyelinated axons share a common invagination of the membrane of the Schwann cell. They are then separated from each other by processes of cytoplasm and finally segregated in individual Schwann cells as a result of repeated Schwann cell division. During myelination the mesaxon forms first loose whorls and then tight spirals around the axon. Compact myelin first appears focally in the outer layers of the spiral. Non-compacted areas appear to persist as the Schmidt-Lantermann clefts; it is suggested that these are areas of growth for the expanding sheath. Redundant folds of myelin are described and discussed.