Vesicle- and collagen-mediated calcification in the turkey leg tendon

The distribution, organization, and orientation of apatitic mineral associated with vesicles and collagen in normal calcifying turkey leg tendon have been examined to determine inorganic-organic interactions in this vertebrate tissue. The study utilized selected area electron diffraction and bright or dark field transmission electron microscopy of thin sections (approximately 70 nm) treated by anhydrous means. Bright field microscopy revealed crystals randomly disposed in radial clusters related to vesicles or highly oriented related to collagen. Dark field images of the crystal c-axis were localized within some hole zone regions of collagen but were generally sparse in vesicles. Images of mineral a,b-axes were associated with vesicles and also appeared in collagen hole zones complementary to those containing c-axis images. Where mineralization of vesicles occurred adjacent to that of collagen, bright and dark field images provided suggestive evidence that the crystals could be contiguous at some sites, but the bulk of extracellular mineralization was spatially discontinuous. The varying disposition of mineral in vesicles and collagen is likely indicative of distinct mechanisms of nucleation in these components, the complementary orientation of mineral c- and a,b-axes in collagen may provide insight into the structure and organization of the protein, and the distribution of vesicle-mediated mineralization appears to contribute to apatite accumulation between collagen fibrils.