Peripheral nerves in shiverer----dystrophic mouse chimeras: evidence that a non-Schwann cell component is required for axon ensheathment in vivo.

Peripheral nerves in dystrophic mice express multiple axon ensheathment abnormalities. If an intrinsic deficiency expressed within the Schwann cells themselves were to account for this neuropathy, then such cells, existing in a chimera preparation, would be expected to express the same ensheathment abnormalities, whereas a coexisting population of non-dystrophic Schwann cells should not be similarly affected. The genotype of myelinated Schwann cells in shiverer----dystrophic chimera was established with immunocytochemical techniques. Shiverer myelin lacks the P1 component of myelin basic protein (MBP), whereas dystrophic myelin appears to contain normal levels of MBP. No correlation between the ensheathment characteristics of the chimera spinal roots and the genotype of the local Schwann cell population was found; both dystrophic Schwann cell populations expressing normalized ensheathment characteristics and shiverer Schwann cells failing to respond to the local presence of naked axons were observed. These results require that a defective extra Schwann cell component is involved in the pathogenesis of the dystrophic neuropathy. Moreover, the normal realization of that component appears to be a necessary prerequisite for shiverer Schwann cells to achieve full ensheathment competence. Although a definitive identification of the cell type(s) that expresses the dy gene locus has not been achieved in this chimera preparation, the observations are consistent with defective endoneurial fibroblast function.