Transport of epidurally applied horseradish peroxidase to the endoneurial space of dorsal root ganglia: A light and electron microscopic study

The route by which an epidurally applied macromolecule might reach the endoneurial space of spinal nerve roots was assessed with light and electron microscopy in a pig model established to explore the pathophysiology of disk herniation. Horseradish peroxidase (HRP) dissolved in saline was infused epidurally. Animals were sacrificed after 5 minutes (n = 5) or 30 minutes (n = 5). Two control animals received only a saline infusion and were sacrificed after 30 minutes. Nerve root specimens were collected, fixed, and exposed to the HRP substrate, 3.3'-di-amino-benzidine (DAB). The distribution of HRP reaction product in the nerve tissue was studied with light and electron microscopy. In 5-minute specimens, HRP was found in epidural and intradural vessel walls. At the nerve root level, HRP was detected in meningeal membranes but was not seen in periaxonal space. In addition to engaging the outer cell layers of the dorsal root ganglion (DRG) capsule, HRP was detected as a gradient among the peripherally located nerve cell bodies and sometimes among the emerging afferent axons. The 30-minute group demonstrated similar findings. The results confirm that HRP can reach the periaxonal spaces of lumbar DRG within 5 minutes after epidural application. Although the transport mechanism is not fully understood, the DRG may constitute an anatomical location allowing epidurally applied macromolecules entrance to the endoneurial space, either by direct diffusion or via vascular transport. The demonstrated transport route may have implications in the pathophysiology of sciatica in conjunction with lumbar disc herniation.