The sensitivity of the myelin sheath, particularly the Schwann cell-axolemmal junction, to lowered calcium levels in cultured sensory ganglia

Light and electron microscopic observations were made on myelinated cultures of sensory ganglia maintained for 2–12 days on media with low calcium (0.025–0.050mM) or normal calcium (1.5–2.0mM) levels. After 18 h at low calcium levels, lengthening of the node of Ranvier and apparent fluid accumulation between the axon and the myelin sheath were observed in the light microscope. Electron microscopic observations indicated that these changes began with an accumulation of fluid between the Schwann cell terminal loops in the paranodal region, shrinkage and stretching of the terminal loops, and eventual breaking of the junction between the loops and the axolemma. Accompanying these changes was substantial fluid accumulation in the periaxonal space along much of the internode. These changes were found to be slowly reversible upon return to media with normal calcium levels. These observations are discussed in relation to the known properties of various types of intercellular junctions and the physiological properties of the myelinated nerve fiber. It is suggested that the unique structural characteristics of the Schwann cell-axolemmal junction in the paranodal region may relate to the presumed functional requirements of saltatory conduction,i.e., adhesiveness for the maintenance of myelin form and/or sealing of the periaxonal space from the general extracellular space, but without the establishment of low resistance channels between Schwann cell and axon.