Fine structure and myelination of the developing human vagus nerve

The development of the cervical part of the vagus nerve was studied by electron microscopy in 17 human embryos and fetuses from 18 to 220 mm crown-rump (CR) length, approximately 7--23 postovulatory weeks old. By the end of the embryonic period proper, processes of neurolemmocytes are beginning to invade the nerve bundles of the vagus. Early in the fetal period, the nerve fibres become more regularly arranged, the lemmocyte/axon ratio is diminishing, the neurolemmocytes are dividing, and the connective tissue elements are becoming differentiated. By 14 postovulatory weeks (120 mm CR), examples of a 1:1 lemmocyte/axon ratio are observed, and the spiral development characteristic of myelination is beginning. By 17 weeks, myelinated fibres at varying phases of myelinogenesis are apparent, and desmosome-like junctions in the opposed plasma membranes are seen where lemmocyte processes form a mesaxon. By 23 postovulatory weeks, the diameter of the axons varies from 0.2 to 3.5 micrometer, and the vagus shows its mature structure, although nerve fibres at the beginning of myelin formation are still visible.