Evidence for a distinct water-rich layer surrounding collagen fibrils in articular cartilage extracellular matrix

Bovine articular cartilage was vitrified by high-pressure freezing. On the one hand vitrified samples were cryosectioned and investigated by cryoelectron microscopy in an unstained frozen hydrated state. On the other hand, they were freeze substituted in pure acetone, ethanol, or methanol, respectively, and subsequently embedded in Epon. Ultrathin Epon sections were poststained with uranyl acetate and lead citrate. The resulting ultrastructural representation was different for every protocol. The evaluation of the combined results provides evidence for a distinct water-rich layer surrounding collagen fibrils in articular cartilage extracellular matrix, which has not been recognized before. The possible composition and function of this layer is discussed.