In vivo and in vitro observations on laminin production by Schwann cells

Recent reports on the dystrophic mouse mutant suggest that the prominent extracellular matrix component of peripheral nerve tissues plays an important role in peripheral nerve development. We have examined the disposition of two prominent extracellular matrix components, fibronectin and laminin, both in mature peripheral nerve in vivo and in an in vitro system that allows study of Schwann cells in various functional states. In frozen sections of whole nerve, staining with antibodies to fibronectin and laminin shows that fibronectin stains throughout the endoneurium while laminin staining is restricted to regions known to contain basal lamina, particularly the basal lamina of each ensheathing Schwann cell. Tissue culture studies indicate that fibronectin staining at the light microscopic level is a reliable marker for fibroblasts (and not Schwann cells) in culture; conversely, antibodies to laminin stain components related to the Schwann cell surface but not components related to fibroblasts. Unexpectedly, Schwann cells in culture produce laminin at all stages in development, whether in contact with axons or not. As Schwann cells in culture begin to ensheathe axons, punctate regions of laminin on their surfaces become confluent. After ensheathment is completed, a continuous line of staining is found in the region of the Schwann cell basal lamina. It has been established that Schwann cells produce a basal lamina only when in contact with axons. Therefore, the production of laminin appears to be necessary but not sufficient for basal lamina formation. The constancy of laminin production by Schwann cells provides a reliable basis for distinguishing between Schwann cells and fibroblasts in tissue culture.