Electron microscope observations on the changing relationships between unmyelinated axons and Schwann cells in human fetal nerves.

The superior rectus oculi muscle from human fetuses of 5, 9.2, 12 and 24 cm crown-rump length (equivalent to 10, 12, 15 and 23 weeks respectively) and a finger from a 9.5 cm crown-rump length (12 weeks) human fetus were examined by electron microscopy. Very simple relationships between Schwann cells and large bundles of axons were found in the superior rectus muscle of the 5 cm fetus and in the large nerve bundles of the fetal finger. Absence of collagen fibrils in the vicinity of these nerve bundles was also noted. In the superior rectus muscles of the older fetuses the complexity of the Schwann cell/axon relationship increased dramatically, the effect being to envelop single axons in Schwann cell processes. This was also the case in the smaller elaborate bundles in the fetal finger which lay adjacent to move primitive, simply invested nerve bundles. In the examination of near serial sections of Schwann cell/axon bundles it was found that the Schwann cell processes, originally likened to a curtain hanging in fluted folds and slightly twisted, was also badly torn and tattered into longitudinal strips of uneven length. In near serial sections, dramatic changes have been found in the arrangements of Schwann cells and axons. Axons may be singled out and separately invested by the Schwann cell processes, but only for short distances. Lateral transfer of axons from one nerve bundle to another may occur, and loss of axons is apparently not uncommon. The same axon may be of very different diameters at different points along its length.