Schwann cells in peripheral nerves secrete a layer similar to the basal lamina called the lamina externa, boundary layer, or basement membrane. In this micrograph, the lamina externa surrounds the Schwann cells that wrap around both myelinated and unmyelinated axons in a mixed nerve from cat pericardium. Arrows indicate several examples of the outer limit of the basal lamina. Myelinated axons can be distinguished from unmyelinated ones by presence of repeated dark radial rings. Figure 26 from Chapter 1 (The Cell Surface) of 'The Cell, 2nd Ed.' by Don W. Fawcett M.D. A PDF copy of the accompanying chapter is available on the ASCB's BioEDUCATE website.