Ultrastructural changes in spinal nerve roots induced by autologous nucleus pulposus

Study Design. Ultrastructural changes were analyzed by transmission electron microscopy in nerve roots exposed to autologous nucleus pulposus experimentally. Objectives. To assess if ultrastructural changes were present in areas with no light microscopic changes in nerve roots exposed to autologous nucleus pulposus in a pig model. Summary of Background Data. Previous analyses have shown that there is focal nerve fiber damage in nerve roots exposed to autologous nucleus pulposus in the pig. These changes could not fully explain the reduction in nerve conduction velocity seen in the same nerve roots. In the present study, the parts of the nerve roots that did not display breakdown of axons or myelin sheaths at the light microscopic level were analyzed regarding ultrastructural changes. Methods. In a previous study, nucleus pulposus was harvested from a lumbar disc and placed epidurally onto the cauda equina at the sacrococcygeal level in pigs. Retroperitoneal fat was used as control. After 1, 3, and 7 days, the nerve roots were excised and processed for light microscopy. Parts of the nerve roots that appeared normal at the light microscopic level were further processed for the present electron microscopic examination. Results. Significant ultrastructural changes, such as expansion of the Schwann cell cytoplasm and intracellular edema with vesicular swelling of the Schmidt-Lanterman incisures, were observed in nerve fibers with normal axons. Although present after nucleus pulposus and control application, the changes were more pronounced after the application of nucleus pulposus. Conclusions. Epidural application of autologous nucleus without any pressure may induce not only nerve function impairment but also axonal injury and significant primary Schwann cell damage with vesicular swelling of the Schmidt-Lanterman incisures. However, because axonal and Schwann cell changes affected only part of the nerve fibers, further causes of the impaired nerve conduction need to be determined.