Ingrowth of nerves and blood vessels is regarded as a part of disc degeneration. Schwann cells, which guide the development and regeneration of nerves both in the central and peripheral nervous system, have recently been located in degenerated human discs. The purpose of this study was to investigate by immunohistochemical methods, whether or not there was nerve, Schwann cell and blood vessel ingrowth in experimentally induced acute (3 weeks) and subacute (3 months) annular tears; and to study the detailed topography of such nerve ingrowth when present. Ingrowth of nerves and Schwann cells has so far not been studied in experimentally induced acute annular tears. A stab incision with a scalpel blade was made in two adjacent lumbar discs in adult sheep (n = 9). The L2–L3 discs were injured superficially, whereas in L3–L4 discs the incision reached the nucleus pulposus (full-thickness injury). The animals were sacrificed three weeks (acute injury, n = 3) or three months (subacute injury, n = 6) post-operatively. Discs were immunoassayed for general neuronal markers (protein gene product 9.5, PGP 9.5 and synaptophysin, SYN), Schwann cell marker (glial fibrillary acidic protein, GFAP) and endothelial marker (CD 31). Nerves visualized with all antibodies employed were situated inside the injured area and mainly near amd around blood vessels although free nerve endings could be seen also. PGP 9.5-immunopositive nerves were evident in every sample. Immunoreactivity to SYN and GFAP was not observed in control discs. Such immunoreactivity appeared already at 3 weeks (acute injury) in a few samples, and was present in all samples at 3 months (subacute injury). For the superficial injury, nerves did not penetrate deeper than in control discs until at 3 months. With more extensive annular injury, mean values for both nerve and Schwann cell penetration were higher. Blood capillaries were visible deeper in the inner annulus than neural tissues immunopositive to PGP 9.5, GFAP or SYN. The results suggest an ingrowth of nerves and Schwann cells with time, following an injury of the annulus fibrosus. A trend for deeper penetration already in the acute stage, but only for deep lesions reaching all the way to the nucleus pulposus, may be suggested. Such ingrowth, also previously reported in degenerated human discs, may relate to mechanisms of low back pain, originating in the intervertebral disc.
Nerves are more abundant than blood vessels in the degenerate human intervertebral disc
