Study design: The innervation of the anulus fibrosus of human macroscopically normal intervertebral discs from five patients was investigated immunohistochemically.
Objectives: Immunoreactivity to general nerve markers (synaptophysin and protein gene product 9.5) and to neuropeptides (substance P and C-flanking peptide of neuropeptide Y) was studied.
Summary of background data: In the lumbar disc of a newborn, free nerve endings have been demonstrated in the outer layers of anulus fibrosus. In degenerated and herniated discs, nerve structures have been shown to penetrate deeper into the anulus fibrosus. There are only a few studies on the innervation of normal adult intervertebral disc tissue.
Methods: Thin frozen sections of human normal lumbar intervertebral disc tissue were immunostained for general nerve markers and neuropeptides.
Results: Synaptophysin and protein gene product 9.5 immunoreactive nerve structures were observed penetrating 3.5 mm and 1.1 mm into the anulus, respectively. Immunoreactivity to C-flanking peptide of neuropeptide Y and substance P were observed at a maximum depth of 0.9 and 0.5 mm in the anulus, respectively. Antibodies to the former have been used to study sympathetic nerves, whereas substance P is a transmitter present in sensory nerves.
Conclusions: In anulus fibrosus samples from macroscopically normal discs, a general marker for nerve endings can be found at a depth of a few millimeters, whereas neuropeptide markers show nerves only in the outermost layers of the anulus fibrosus. This absence of demonstrable nerves in deeper anulus fibrosus in normal discs is probably not a methodologic artifact, because blood vessels have also been demonstrated only at the disc surface. It is, however, possible that neuropeptide nerves also penetrate to a depth of a few millimeters, but that methodologic limitations permit the visualization of only the neuropeptide nerves closest to the disc surface. The results of the present study lend support to previous suggestions that, except at the surface, a normal intervertebral disc is almost without innervation.