Identification and distribution of type VI collagen in tendon fibrocartilages

Scructural and compositional variations are marked in tendon fibrocartilage s, which appear at the insertion to bone and in areas subjected to compress ive plus frictional loading regime. We are interested in characterizing the extracellular matrix adaptations in these areas, in an attempt to understa nd cellular responses to biomechanics. In this work, we have applied immuno cytochemistry and an ATP treatment for the ultrastructural identification o f type VI collagen, to tendons subjected to compressive forces in different species. Immunocytochemistry, after testicular hyaluronidase or pepsin dig estion, revealed the presence of type VI collagen in tensional and compress ive areas of the plantaris longus tendon of the bullfrog, in the deep flexo r tendon of dogs and rabbits, in the calcanear tendon and the suprapatela o f rats and in che gastrocnemius tendon of chickens. In each tendon, the ten sional region showed a weak reaction, restricted to the collagen fiber surf ace. However, the compression region was especially rich in type VI collage n, which accumulates in the interfibrillar spaces and is concentrated aroun d the fibrochondrocytes. Intense reaction was also found in the paratenon. The ATP treatment not only allowed for the detection of the typical ladder- like aggregates of type VI collagen in the same areas identified by immunoc ytochemistry, but also demonstrated that type VI collagen forms a microfibr illar network around the fibrochondrocytes. Besides the function of organiz ing groups of collagen fibrils, type VI collagen seems to assemble the peri cellular matrix in tendon fibrocartilages, perhaps through physical interac tions with the large proteoglycans that: concentrate in the same area.