On the occurrence of Schwann cells within the normal central nervous system.

The presence of Schwann cells and P.N.S. myelin are reported in subpial areas of apparently normal spinal cord from one control rabbit, two experimental rabbits and one experimental guinea pig. These P.N.S. elements exerted no adverse effects upon local C.N.S. components. The occurrence of ectopic Schwann cells in the normal C.N.S. has also been reported elsewhere in studies on normal human spinal cord tissue. The propensity for Schwann cells to reside in the normal C.N.S. of several species makes it necessary for experiments and hypotheses on the aetiology of Schwann cell invasion into the abnormal C.N.S. to take the present phenomenon into consideration.