The association of proteogtycans with type I collagen fibrils in skin, tendon, cornea and bone has been determined by electron microscopy using an electrondense dye, Cupromeronic blue, in the critical electrolyte concentration mode, backed up by biochemical analysis and digestion by hyaluronidase or keratanase. A major proteoglycan of the soft tissues, containing dermatan sulphat, is shown to be regularly and orthogonally arranged at the surface of the fibrils. Uranyl acetate counterstaining revealed that the main specific binding site is the ‘d’ band, which previous work indicated is very close to the initial site of calcification of type I collagen fibrils. Bone, deminer-alized by a ‘non-aqueous’ technique which preserves the proteoglycan in the tissue does not contain orthogonal arrays; the interfibrillar proteoglycan filaments are oriented parallel to the fibril axis. The main proteoglycan in bone is chondroitin sulphate-rich. It is suggested that dermatan sulphate proteoglycan plays a role in preventing soft connective tissues from calcifying.

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