This low magnification EM image shows axons in a peripheral nerve bundle in transverse section. This is a similar sample to HA13. The heavily stained structures are myelin. These are elaborations of the Schwann cell membrane that insulate the axon to facilitate rapid conduction of the action potential. On some axons, you can see the nucleus of the Schwann cell that provides the myelin. At the lower right you can see a Schwann cell cradling and protecting 12 axons but without myelin. Below 1 micron, axons are not myelinated. The space between axons is filled with connective tissue called endoneurium.