Disc morphology in health and disease.

The morphology of the intervertebral disc is dependent on the type of components present and the manner in which they are assembled. This, in turn, will determine how the tissue carries out its primary physiological functions of load bearing and allowing movement in all directions of the otherwise rigid spine. The disc starts its life in an ordered fashion, with the outer annulus fibrosus consisting of a series of regular concentric bundles of collagen fibres around the central gelatinous nucleus pulposus. With advancing age there is increased complexity of lamellae, with more bifurcations, interdigitations and irregularity in number and size of lamellar bands, probably as a result of the disc cells' synthetic response to variations in incident loads. This change in organization itself will lead to altered load bearing, hence possibly establishing a self-perpetuating cycle of disruption to disc morphology, which, once started, may be irreversible. There are also alterations to cell organization with disease and degeneration. There may be increased cell number, with clusters of cells forming by cell proliferation, or increased cell death, whether by necrosis or apoptosis, in degenerate or deformed discs such as are found in spinal deformities; for example, scoliosis. Any change in cell number is likely to alter the nutritive requirements and concentration gradients of both nutrients and metabolites. The normally avascular disc in the healthy adult can become increasingly vascularized and innervated with degeneration and disease. This may lead to an increased supply of oxygen and nutrients to the disc, but can also introduce other cells types and molecules such as cytokines and growth factors.