Substance P-immunoreactive and protein gene product 9.5-immunoreactive nerve fibres in bone marrow of rat coccygeal vertebrae.

Previous investigations have focused mainly on the nerve terminals in soft tissues surrounding the vertebrae to determine the source of spinal pain. Our study on rat coccygeal vertebrae compared intramedullary immunostaining for substance P with that of a more generally distributed neural marker, protein gene product 9.5, to suggest another source of spinal pain. Free intramedullary fibres staining for protein gene product 9.5 were rare compared with the abundant staining associated with intramedullary vessels. Shortly after entering the marrow with the nutrient vessels, substance P-immunoreactive fibres parted from the vessels and then proceeded longitudinally and terminated on the end-plates. Other (although fewer) substance P-immunoreactive fibres entered the marrow at enthetic aspects of the vertebrae. The presence of free substance P-immunoreactive fibres innervating endplates and penetrating entheses suggests that they may represent a novel source of spinal pain.