Myelination of the human fetal phrenic nerve

The formation of the myelin sheath of the human phrenic nerve has been studied by electron microscopy in fetuses aged 15, 17, and 23 postovulatory weeks. At 15 weeks, the phrenic nerve is composed mainly of large bundles of axons surrounded by lemmocyte processes. However, fibres at the beginning of myelination are present and show one or one and a half turns of lemmocyte process. At 17 weeks, an increasing number of myelinating fibers can be noted, and the formation of compact myelin and laminated perineurium is detectable. The phrenic nerve presents a mature form by 23 postovulatory weeks. At this time many axons have acquired a thick, compact myelin sheath. However, fibres at early stage of myelin formation are still visible.