Myelin maintenance by Schwann cells in the absence of axons

Formation and maintenance of myelin sheaths in the peripheral nervous system are regulated by unknown molecular interactions that are thought to depend upon physical contact between Schwann cells and axons. However, recent studies describing axons surrounded by two concentric myelin internodes in the superior cervical ganglion (SCG) of normal rodents have demonstrated that the outer myelin internodes are maintained without physical contact with the axon. To determine whether the centrally enclosed axon has a trophic effect in maintaining these remote outer internodes, we have produced axonal degeneration by surgical or chemical means. The results indicate that maintenance of myelin internodes totally displaced from axonal contact depends neither upon the presence of the axon nor on diffusible axonal factors. A further implication of these studies is that myelin breakdown during Wallerian degeneration is regulated by a positive signal which originates in degenerating nerves, rather than solely by loss of axonal trophic substances.