A comparative electron microscopic study of apatite crystals in collagen fibrils of rat bone, dentin and calcified turkey leg tendons

Crystal-collagen relationships in calcified turkey leg tendons and cortical bone and dentin of the rat were studied by bright field and selected-area dark field electron microscopy. The latter imaging technique enables the specific and direct visualization of apatite crystal sizes and their crystallographic orientations within collagen fibrils. Cortical bone possessed the longest mean c-axial length (170 +/- 50 A), then the tendon (142 +/- 43 A) and the smallest was dentin (110 +/- 30 A). Crystallographic orientations of apatite were found to alternate between a,b- and c-axial planes along the axial period of longitudinally sectioned collagen. This distribution of apatite may reflect a crystal alignment with collagen molecules as they spiral in superhelical fashion along the long axis of the collagen fibril. Apatite crystals were localized within both the gap and overlap zones of collagen fibrils even at very early stages of mineralization. The relative amounts of mineral within single collagen periods were determined as a function of electron absorbency. In the tendon at the onset of mineralization 80% of the mineral was located in the gap zone and 20% in the overlap zone; with further mineralization these relative amounts changed to 55% in the gap zone and 45% in the overlap zone. This 55/45% ratio observed in the heavily mineralized tendon was also observed in both cortical bone and dentin. The implications of these findings are discussed in view of collagen molecular ordering and the spread of apatite along collagen fibrils.