Collagen fibrils are differently organized in weight-bearing and not-weight-bearing regions of pig articular cartilage

The magnetic resonance (MR) appearance of the weight-bearing ("loaded") and not-weight-bearing ("unloaded") regions in T-2-weighted images of pig arti cular cartilage is different. On the hypothesis that this difference may be ascribed, at least in part, to a different collagen fibre organization in the two regions, this organization was studied using biochemical, histologi cal, and X-ray diffraction methods. While the mean concentrations of collag en and of its crosslinks were the same in the two regions, a regular small angle X-ray diffraction pattern was observed only for the habitually "loade d" tissue. It was also seen by light microscopy that the four typical funct ional zones were well displayed in the "loaded" cartilage whereas they were not; clearly depicted in the "unloaded" tissue. Collagen presented a high concentration of fibrils forming an intricate and dense meshwork at the sur face of both "loaded" and "unloaded" cartilage. A second zone of high colla gen concentration was present at the upper layer of the deep zone of "loade d" cartilage. By contrast, this lamina of highly concentrated fibrils was l acking in "unloaded" cartilage and collagen fibrils appear thinner. Our stu dy proves that the organization of collagen fibres is different for the "lo aded" and "unloaded" regions of articular cartilage. It also suggests that this different organization may influence the MR appearance of the tissue. J. Exp. Zool. 287:346-352, 2000. (C) 2000 Wiley-Liss, Inc.