Development of unmyelinated fibers in peripheral nerve--an immunohistochemical and electronmicroscopic study.

The development of the unmyelinated fibers in the mouse sciatic nerve was studied with immunohistochemical and electronmicroscopic techniques. Glial fibrillary acidic protein (GFAP)-immunostaining showed fainty-positive cells in transverse sections on day 2. Staining intensity and numbers of stained cells increased with age and both reached the maximum around day 30. Immunoelectronmicroscopy for GFAP revealed that positive staining was confined to the cytoplasm of Schwann cells of the unmyelinated fibers. In contrast vimentin-immunoreactivity was detectable as early as day 0. Schwann cells of both myelinated and unmyelinated fibers were positive. Electronmicroscopic study showed that Schwann cell families contained both larger and smaller axons on day 0. By day 5 majority of the larger axons were separated from Schwann cell families, which then contained only smaller axons. Ensheathment of smaller axons by Schwann cell processes were completed around day 30. These results showed that GFAP was expressed in Schwann cells of the unmyelinated fibers along with the differentiation and maturation of the unmyelinated fibers.