ABSTRACT
The annulus fibres are composed of types I and II collagen (Urban, 1996). The inner annulus is primarily type II collagen, while the outer annulus is mostly type I collagen (Eyre and Muir, 1976). The fibres constitute 16 per cent of the annulus volume and are oriented 30° to horizontal in a criss-cross pattern between lamellae. The histochemistry of the annulus changes from the central to peripheral regions. The central portion has a higher glycosaminoglycan content and lower collagen content than the periphery. This gives the central portion a gel-like appearance with more hydrostatic shock-dispersing properties and the peripheral more tensile properties. The fibres in the central portion of the annulus fibrosus connect to the cartilaginous endplates, while the peripheral (Sharpey’s) fibres connect to the vertebral bodies. The attachment of Sharpey’s fibres to the vertebral bodies is much stronger than where the fibres attach to the endplates in the central annular region (White and Panjabi, 1990). Peripheral annular cells appear spindle shaped, resembling fibrocytes of tendons, while central annular cells appear rounded and are located in the lacunae of chondrocytes (Holm, 1996). These cells maintain the proteoglycan content of the extracellular matrix (Johnstone and Bayliss, 1996). Proteoglycan synthesis occurs most in the mid-annulus region (Bayliss et al., 1988).
