Compartmentalization of the matrix formed by nucleus pulposus and annulus fibrosus cells in alginate gel

Intervertebral disc cells cultured in alginate gel are capable of reforming in alginate, a matrix that consists of two compartments: a rim of metabolically active cell-associated matrix and a more abundant, but metabolically less active, further removed matrix. At any one age and in most species, the cell-associated matrix formed by a nucleus pulposus or annulus fibrosus cell cultured in this way is less abundant than that formed by an articular chondrocyte. In both the cell-associated matrix and further removed matrix, the ratio of aggrecan to collagen is significantly higher in the case of nucleus pulposus than of annulus fibrosus, a feature that also distinguishes the matrices of the nucleus pulposus and annulus fibrosus in vivo. Nucleus pulposus and annulus fibrosus cells from older donors show a decreased ability to reform a cell-associated matrix rich in aggrecan. There is, however, some evidence that gene therapy and/or exposure of the cells to defined stimulatory factors can help overcome some of these age-related limitations. This contention is supported by recent evidence that nucleus pulposus and annulus fibrosus cells from adult donors can be manipulated to form, using the recently developed alginate-recovered chondrocyte system, a resilient tissue that bears many of the characteristics of the tissue in which these cells reside in vivo.