Biological Responses of the Immature Annulus Fibrosus to Dynamic Compression in a Disc Perfusion Culture

Mechanical stimuli participate in disc development and remodelling. However, the effects of mechanical load on the immature annulus fibrosus (AF) are largely unclear. This study aimed to investigate how the immature AF responded to dynamic compressive magnitude and duration. Immature porcine discs were bioreactor-cultured for 7 days and then dynamically compressed at various magnitudes (0.1, 0.2, 0.4, 0.8 and 1.3 MPa at a frequency of 1.0 Hz for 2 h/day) and durations (1, 2, 4 and 8 h/day at a magnitude of 0.4 MPa and a frequency of 1.0 Hz). Non-compressed discs were used as controls. The immature AF tissue was analysed for histology, gene expression (aggrecan, collagen I, ADAMTS-4, MMP-3, TIMP-1 and TIMP-3), biochemical content of glycosaminoglycans (GAG) and hydroxyproline (HYP) and aggrecan immunohistochemical staining. In the lower-compressive-magnitude groups (0.1, 0.2 and 0.4 MPa), the immature AF showed an up-regulation in the expression of matrix genes, GAG and HYP content and aggrecan deposition. In the compression duration groups, the GAG and HYP content and aggrecan deposition declined to a minimum in the 8-hour group, in which a catabolic gene expression profile was found. In conclusion, this study indicated that the effects of dynamic compression on the immature AF are magnitude and duration dependent and that catabolic remodelling within the immature AF can be induced by high compressive magnitudes and long compressive durations.