Articular cartilage: collagen II-proteoglycans interactions. Availability of reactive groups. Variation in birefringence and differences as compared to collagen I.

Intrinsic (IB) and form or textural birefringence curves (FBC) were obtained measuring optical path differences in non stained sections of articular cartilages. Differences were not detected when comparing IB and FBC of the articular surface with those of the intermediate zone (Collagen II), but were found when these curves were compared with those of tendons (Collagen I). A detailed description of arrangements of collagen fibers are reported with no confirmation of Gothic arches. Hyaluronidase and papain digestion caused a decrease in birefringence of the cartilage supporting the conclusion that acid proteoglycans (APG) present molecular orientation in those structures. Histochemical data and histophotometric measurements confirmed that in articular cartilage there are variations in APG concentration and complexation with Collagen II from the articular surface (AS) down to the bone zone, AS displaying much more available -NH3+ groups. Molecular order in the articular cartilage optimizes the state of aggregation in its structures, and is the basis for its piezoelectric and mechanical properties.