Fibroblasts promote Schwann cell basal lamina deposition and elongation in the absence of neurons in culture.

We and other investigators have repeatedly observed that, although purified populations of Schwann cells cultured in the absence of neurons immunostain for some basal lamina constituents, assembled basal lamina is not seen ultrastructurally. In this study we show that purified rat Schwann cells cultured with either fibroblasts or diffusible substances from fibroblasts acquire basal lamina; this basal lamina is most robust and continuous when Schwann cells and fibroblasts are cultured together. The diffusible fibroblast factor(s) is ascorbate-dependent and heat-sensitive and exhibits a molecular weight greater than 30-50,000. The addition of either a combination of basal lamina components or purified laminin also results in the deposition of lengths of basal lamina. Extracellular matrix elaboration promoted by fibroblast factors is linked to dramatic changes in the morphology of Schwann cells; the cells become elongated and aligned with respect to each other. The addition of purified laminin partly mimics this effect. These changes may represent an early response of the Schwann cell to the acquisition of basal lamina preceding further differentiation in vivo. Our observations resemble those reported for other epithelial cell-fibroblast interactions and thus reaffirm the identity of the Schwann cell as an epithelial cell sensitive to the influence of mesenchymally derived factors. We propose that fibroblast stimulation of Schwann cell extracellular matrix deposition probably occurs in normal development with axons and in abnormal situations without axons, such as peripheral nerve injury and tumor formation.