Staying connected: structural integration at the intervertebral disc-vertebra interface of human lumbar spines

To investigate the microscopic fibrous integration between the intervertebral disc, cartilage endplates and vertebral endplates in human lumbar spines of varying degrees of degeneration using differential interference contrast (DIC) optics. Weakness at these junctions is considered to be an important factor in the aetiology of disc herniations.

Magnetic resonance images (MRIs) of cadaveric lumbar spines were graded for degeneration and motion segments from a range of degenerative grades isolated and bisected sagittally. Following fixation and decalcification, these were cut into segments containing anterior or posterior annulus fibrosus or nucleus pulposus. The segments were cryo-sectioned and sections visualised using both standard light and DIC microscopy.

Detachment at the interface between the disc and vertebrae increased with greater degenerative grade (from 1.9 % in Grade I to 28 % in Grade V), especially at the boundary between the cartilage and vertebral endplates. DIC microscopy revealed the fibrous organisation at the IVD-cartilage endplate interface with structural features, such as annular lamellae branching and nodal insertions in the nucleus pulposus region; these have been previously observed in ovine spines, but were less uniform in humans. Structural integrity of the IVD and cartilage endplate was also lost with increasing degeneration.

This preliminary study shows that microscopic structural features may act to maintain attachment between the IVD and CEP in the human spine. Loss of structural integrity in this region may destabilise the spine, possibly altering the mechanical environment of the cells in the disc and so potentially contribute to the aetiopathogenesis of IVD degeneration.