In case of the peripheral nerves, Schwann cells that are wrapped around the nerve fibers produce a fatty substance called myelin. Myelin insulates the axons. Endoneurium, perineurium, and epineurium are layers of connective tissue that cover the individual axons, fascicle, and the entire nerve, respectively. It must be noted that both myelinated and unmyelinated axons have an endoneurium.