Three dimensional analysis of Schwann cells associated with unmyelinated nerve fibres in human sural nerve.

In a cross section the profile of one or several unmyelinated axons is embraced by profiles of one or several Schwann cells, all surrounded by a basal lamina. Reconstructions demonstrate that this complex structure is the result of overlap and regrouping of contiguous Schwann cells and only to a lesser extent to branching of Schwann cells. More than half the 36 Schwann cells reconstructed did not branch within the 200 micrometer analysed, and one fourth had two branches. Notwithstanding the regrouping of Schwann cells and axons at different levels of the nerve, the number of Schwann cell subunits in a single cross section is a suitable gauge for the number of Schwann cells. The single nucleus is situated midway between the ends of the Schwann cell. The length of the Schwann cells ranges from 220 to somewhat more than 400 micrometer; the thickness is 0x2-0x5 and in the region 2-5 micrometer. Collagen pockets are outpouchings of the Schwann cell which rarely extend over more than 6 micrometer along the Schwann cell; they are spaced at average intervals of 20 micrometer. Outpounchings devoid of collagen pockets have a similar length but occur more rarely. Abnormally dense cytoplasm, when present, is seen throughout the entire length of the Schwann cell.