Viscoelastic properties of intervertebral disc cells - Identification of two biomechanically distinct cell populations

Study Design. A combined experimental and theoretical mechanical study to q uantify the mechanical properties of living cells of the porcine interverte bral disc. Objectives. To quantify zonal variations in the mechanical properties and m orphology of cells isolated from the intervertebral disc. Summary of Background Data. Cellular response to mechanical stimuli is infl uenced by the mechanical properties of cells and of the extracellular matri x. Significant zonal variations in intervertebral disc matrix properties ha ve been reported. No information is currently available on the correspondin g regional variations in the mechanical properties of intervertebral disc c ells, despite evidence of significant differences in cellular phenotype and biologic response to loading. Methods. The micropipette aspiration test was used in combination with a th ree-parameter viscoelastic solid,:model to measure the mechanical propertie s of cells isolated from the anulus fibrosus, transition zone, and nucleus pulposus. Results, Intervertebral disc cells exhibited viscoelastic solid behaviors. Highly significant differences were observed in the morphology, cytoskeleta l arrangement, and biomechanical properties of the nucleus pulposus cells a s compared with anulus fibrosus or transition zone cells. Cells of the nucl eus pulposus were approximately three times stiffer and significantly more viscous than cells of the anulus fibrosus or transition zone. Conclusions. The findings of this study provide new evidence for the existe nce of two biomechanically distinct cell populations in the intervertebral disc, These differences in mechanical behavior may be related to observed d ifferences in the cytoskeletal architecture between these cells, and may fu rther play an important role in the development, maintenance, and degenerat ion of the intervertebral disc.