Three-dimensional ordered distribution of crystals in turkey tendon collagen fibers

The organization of apatite crystals and collagen fibrils in mineralized turkey tendon has been studied by electron microscopy and electron diffraction. To minimize artifactual distortions the tissue was examined, for the first time, as isolated fibrils in an aqueous environment of vitreous ice, as well as in conventionally prepared sections. The electron micrographs show that the plate-shaped apatite crystals are arranged in parallel arrays across the collagen fibrils. This provides direct evidence for highly asymmetric assembly in collagen fibrils, and, indeed, the fibrils were observed to be elongated rather than round in cross-section. There is, furthermore, a pronounced tendency for the layers of crystals to be coherently aligned in adjacent fibrils. These observations may also be important for understanding the mechanical behavior of bone at the molecular level, as such extended, aligned aggregates of flat crystals could develop into natural fracture planes in mature bone.