As articular cartilage is avascular, diffusion at a tissue length scale is the primary mode of solute and nutrient transport to its cells. The major extracellular matrix components are water (70–80%), chondrocytes, collagen (10–20%) and proteoglycans (5–10%) bearing sulfated glycosaminoglycans (GAG) [1]. Electron microscopy studies have shown that articular cartilage can be regarded as having three separate structural zones — superficial, middle and deep. The proportions of the various matrix components vary from the surface to the deep zone in any given joint and the greatest variations in content occur in the GAG content [2]. In addition the collagen fiber alignment varies, with fibers oriented parallel to the articular surface in the superficial zone, randomly oriented in the middle zone and oriented perpendicular to the surface in the deep zone. To a large extent, it is the spatially inhomogeneous composition of articular cartilage and microstructural orientation of its extracellular matrix components that determines the tortuosity of the transport pathway [3]. We therefore hypothesized that the diffusivity profile of a solute through the cartilage depth is inversely related to the GAG content and that the ratio between the axial and lateral diffusivities within each cartilage zone is related to the degree of anisotropy within the zone.
Extracellular matrix changes in response to recombinant human fibroblast growth factor 18 studied in ex vivo cultures of articular cartilage