The findings of this study suggest that anulus fibrosus and transition zone cells undergo a reversible shift in phenotype when cultured in monolayer compared with alginate. These differences suggest that the culture system exerts a strong influence on cell phenotype and may play a role in the response of these cells to biophysical and biochemical stimuli in vitro.

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Cellular response to mechanical loading varies between the anatomic zones of the intervertebral disc. This difference may be related to differences in the structure and mechanics of both cells and extracellular matrix, which are expected to cause differences in the physical stimuli (such as pressure, stress, and strain) in the cellular micromechanical environment. In this study, a finite element model was developed that was capable of describing the cell micromechanical environment in the intervertebral disc. The model was capable of describing a number of important mechanical phenomena: flow-dependent viscoelasticity using the biphasic theory for soft tissues; finite deformation effects using a hyperelastic constitutive law for the solid phase; and material anisotropy by including a fiber-reinforced continuum law in the hyperelastic strain energy function. To construct accurate finite element meshes, the in situ geometry of IVD cells were measured experimentally using laser scanning confocal microscopy and three-dimensional reconstruction techniques. The model predicted that the cellular micromechanical environment varies dramatically between the anatomic zones, with larger cellular strains predicted in the anisotropic anulus fibrosus and transition zone compared to the isotropic nucleus pulposus. These results suggest that deformation related stimuli may dominate for anulus fibrosus and transition zone cells, while hydrostatic pressurization may dominate in the nucleus pulposus. Furthermore, the model predicted that micromechanical environment is strongly influenced by cell geometry, suggesting that the geometry of IVD cells in situ may be an adaptation to reduce cellular strains during tissue loading.

Cells of the intervertebral disc have a limited capacity for matrix repair that may contribute to the onset and progression of degenerative disc changes. In this study, the biosynthetic capacity of cells isolated from specific regions of the porcine intervertebral disc was evaluated in vitro. Using a competitive reverse transcription-polymerase chain reaction technique, gene expression levels for types I and I1 collagen were quantified in cells cultured for up to 21 d in a three-dimensional alginate culture system and compared to levels obtained for cells in vivo. The mechanical properties of cell-alginate constructs were measured in compression and shear after periods of culture up to 16 weeks. Cells from the anulus fibrosus expressed the most type I collagen mRNA in vivo and in vitro, while cells from the transition zone expressed the most type I1 collagen mRNA in vivo and in vitro. Mechanical testing results indicate that a mechanically functional matrix did not form at any time during the culture period; rather, decreases of up to 5o"h were observed in the compressive and shear moduli of the cell-alginate constructs compared to alginate with no cells. Together with results of prior studies, these results suggest that intervertebral disc cells maintain characteristics of their phenotype when cultured in alginate, but the molecules they synthesize are not able to form a mechanically functional matrix in vitro. 0 3001 Orthopaedic Research Society. Published by

The findings of this study provide new evidence for the existence of two biomechanically distinct cell populations in the intervertebral disc. These differences in mechanical behavior may be related to observed differences in the cytoskeletal architecture between these cells, and may further play an important role in the development, maintenance, and degeneration of the intervertebral disc.