Fine structure of human fetal nerve.

Sciatic nerve of 16, 18, 20 and 24 weeks old fetuses was examined electron microscope. Morphologic specificiity and difference between the structure of fetal and mature nerves were demonstrated. The myelination of human sciatic nerve was found to begin after the 16th week of fetal life. From that time the number of myelinated fibers increases progressively and the number of fetal fibers decreases. The time of the end of myelination in man is unknown. The studies of the later period of development and maturation of fetal nerve may give necessary information about the period of the end of myelination. In view of increasing interest in pathogensis of comparable data studies on embryogenesis of human peripheral nerve are indispensable for evaluation of some changes observed in congenital diseases. The ultrastructural observations of human fetal nerve are scarce (3, 6, 7, 10) and they concern only single nerves. The morphological specificity of fetal nerve and characteristic difference between fetal and mature nerves require basic data on different forms of fetal nerve and time of transition of one form into another.