Electron micrograph of a nerve from the mesentery of a rat shows groups of unmyelinated axons wrapped by deeply invaginating Schwann cells in cross section. Surrounding the Schwann cell wrapping is a secreted basement membrane (known as the lamina externa, or boundary layer), similar to the basal lamina produced by epithelial cells. Figure 25 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.