Studies of Schwann Cell Proliferation

In the preceding paper (Salzer et al ., 1980, J. Cell Biol. 84:753-766), evidence was presented that a neurite membrane fraction could be used to stimulate Schwann cell proliferation in culture . In this study, we present evidence that the mitogenic signal by which intact neurites or neurite membranes stimulate Schwann cell proliferation is located at the neurite surface . This conclusion is based on the following observations : (a) stimulation of Schwann cell proliferation by neurons requires direct contact between neurites and Schwann cells, separation of the two cells by a permeable collagen diaphragm 6 ,um thick prevents Schwann cell proliferation ; (b) treatment of intact neurites with trypsin before preparation of neurite membranes abolishes the ability of these membranes to be mitogenic for Schwann cells; and (c) the mitogenic activity ofneurite homogenates is exclusively localized in the particulate rather than the soluble fraction of the homogenate . The mitogenic component on the neurite surface is heat labile, and is inactivated by aldehyde fixation . Preliminary data suggest that the mitogenic effect of neurite on Schwann cells is not mediated by 3',5'-cyclic AMP.