Varicosities in human fetal sciatic nerve fibres.

A morphological study performed on sciatic nerves from 10 fetuses aged 19 to 32 weeks revealed variations in axonal diameter along the length of the fibres but a uniform myelin sheath thickness. This gave the fibres a beaded appearance. The diameter of the axon in the varicosities was up to seven-times greater than that of the intervening axon. The varicosities were separated by distances up to 50 microns. Both myelinated and single unmyelinated fibres had varicosities. Neurofilaments and neurotubules were more densely packed in the axons between the varicosities. The absolute number of filaments and tubules per axon was similar in axons with equal numbers of myelin lamellae but with different diameters, as calculated from transverse sections. The varicosities were observed in all fetuses aged 19 to 24 weeks, but in only one of the two fetuses aged 28 weeks. They were not present in the 32 week fetus. They appear to be a characteristic morphological feature of nerve fibres during early fetal life and can be identified only in teased fibre preparations or in longitudinal sections of the nerve. Their presence explains the bimodal or markedly skewed distribution of myelinated fibre axon diameters that was seen in nerves from young fetuses. It also helps in understanding the discrepancies reported in size patterns between axon diameter and myelin thickness. It is possible that the varicosities may be partially artefactual but their occurrence may imply a particular vulnerability of fetal nerve fibres. Their production may be related to movements of the axoplasmic fluid which is abundant in young fetuses.