Site-specific molecular diffusion in articular cartilage measured using fluorescence recovery after photobleaching

Diffusive transport of solutes is critical to the normal function of articular cartilage. The diffusion of macromolecules through cartilage may be affected by the local composition and structure, which vary with depth from the tissue surface. We hypothesized that the diffusion coefficient of uncharged molecules also varies with depth and molecular size. We used fluorescence recovery after photobleaching (FRAP) to measure site-specific diffusion coefficients of fluorescent dextran molecules (3, 40, 70, and 500 kDa) in porcine articular cartilage. The diffusion coefficients measured using FRAP exhibited an inverse size dependence and were in general agreement with those measured using other techniques. The diffusion coefficients for all molecules varied significantly with depth in a manner that depended upon the size of the diffusing molecule. The diffusion coefficients for the 3 and 500 kDa dextrans were 1.6 and 2.4 times greater, respectively, in the surface zone as compared to the middle and deep zones, whereas the diffusion coefficients of the 40 and 70 kDa dextrans were 0.3 and 0.2 times lower in the surface zone as compared to the middle and deep zones. These differences may reflect variations in the structure and composition of collagen, proteoglycans, and other macromolecules among the zones.