Solute Diffusion in TMJ Disc Measured Using FRAP

Objectives:  The temporomandibular joint (TMJ) disc is a fibrocartilaginous tissue. Due to its avascular nature, diffusive transport of solutes through the extracellular matrix plays a key role in maintaining the normal function of TMJ disc. The diffusion of solutes through TMJ disc may be affected by the local composition and structure. Thus, the objective of this study was to examine inhomogeneous and anisotropic solute diffusion in TMJ disc using fluorescence recovery after photobleaching (FRAP) technique.

Methods:  Sixteen fresh porcine TMJ discs from eight young adult pigs (10~12 months) were used. Tissue slices were prepared from the intermediate, posterior, anterior, lateral, and medial regions of the discs. For each region, diffusion coefficients of fluorescent dextran molecules (4, 40, 70, and 500kDa) were measure in two principle directions which are parallel and perpendicular to TMJ disc, respectively. All FRAP experiments were performed on a Leica confocal laser scanning microscope. Image processing was implemented using custom written code in Matlab to calculate diffusion coefficient.

Results:  The diffusion coefficients measured using FRAP exhibited an inverse size dependence. The diffusion coefficients for all molecules varied significantly with region. These values were significantly higher in the anterior and posterior regions as compared to the intermediate region. Diffusion anisotropy was observed, which was dependent on the region in the disc. Anisotropic diffusion was also dependent on the size of the diffusing molecule, with greatest anisotropy observed for larger molecules.

Conclusion:  Our results showed that the diffusion in TMJ disc is regionally varied and orientation dependent. These differences may reflect variations in the structure and composition of collagen, proteoglycans, and other macromolecules among the regions. Our results suggest that the inhomogeneous and anisotropic characteristics may play a crucial role in diffusion transport of nutrients, signal molecules, and matrix molecules in TMJ discs. (Supported by NIH P20RR-016461 and P20RR-017696).