The role of Schwann cells in the development of human peripheral nerves: An electron microscopic study

Abstract

The Schwann cells of the sciatic nerve and brachial plexuses of four human fetuses aged 12, 14, 16, and 22 weeks of intrauterine life, were studied with the electron microscope. Four stages were recognized in the development of these cells: (a) pseudosyncytial, (b) migration, (c) division of cells and separation of axons, and (d) myelination. During all stages of development of human peripheral nerves, Schwann cells seem to have, as a morphological and perhaps physiological function, the isolation of axons from the surrounding connective tissue elements. During the intermediate stages of development, the main function of Schwann cells appears to be the separation of the axon bundles into smaller and smaller bundles, until one single axon has been finally separated from a bundle. This is accomplished by the continued division of the Schwann cells. After axon isolation has been accomplished, the function of the Schwann cells is myelination (14–16 weeks). All axons, either isolated or in bundles, had a complete or almost complete envelope of Schwann cell cytoplasm. At all stages of development a basement membrane separated the Schwann cell axon complexes from the surrounding mesenchymal tissues. It was suggested that the continual formation of cytoplasmic processes and division of the Schwann cells may be determined, at least in part, by the intimate contact of these cells with progressively enlarging axons.