Expression of alpha-smooth muscle actin in canine intervertebral disc cells in Situ and in collagen-glycosaminoglycan matrices in vitro

The objective of this study was to investigate the presence of a contractil e actin isoform, a-smooth muscle actin, in annulus fibrosus cells in situ a nd in two and three-dimensional cultures. Annulus fibrosus cells were isola ted from healthy adult dogs, serial passaged, and then injected into porous collagen-glycosaminoglycan copolymers consisting of either type-I or type- II collagen, a-Smooth muscle actin was detected in the cells in tissue samp les and in culture by immunohistochemistry. The number of cells and glycosa minoglycan content of the matrices were determined after 1, 7, and 14 days, and the diameters of the specimens were measured every 2 days. Although fe w annulus fibrosus cells in vivo displayed the presence of the a-smooth mus cle actin isoform, most cells in two-dimensional culture demonstrated this phenotype. The contractile behavior of these cells was shown by the cell-me diated contraction of type-I collagen-glycosaminoglycan scaffolds after 8 d ays in culture. Glycosaminoglycan production was not significantly differen t in the seeded type-I matrices than in the unseeded matrices, whereas the seeded type-II matrices had a significant increase in glycosaminoglycan pro duction between days 1 and 14 compared with the unseeded controls. This is the first report of both the expression of the contractile a-smooth muscle actin isoform in intervertebral disc cells and the ability of the cells to contract a collagen matrix. This finding could aid in better understanding the nature of cells in the annulus.