Three-dimensional analysis of mineralizing turkey leg tendon: matrix vesicle-collagen relationships.

The spatial and temporal relationships of mineral deposition between matrix vesicles and type I collagen fibrils have been studied in the turkey leg tendon by electron microscopy of cross sections and serial longitudinal thin sections and by electron tomography of longitudinal thick sections. Serial sectioning and electron tomography allow three-dimensional analysis of spatial relationships, overcoming the problems of missing depth information and over-projection of adjacent structures which exist for two-dimensional projections of isolated sections. These techniques reveal that while mineral deposits within matrix vesicles are found remote from calcifying collagen fibrils, the reverse relationship does not occur; all collagen-associated mineral can ultimately be linked to mineral-laden vesicles. These results suggest a temporal sequence of calcification beginning in matrix vesicles and spreading to adjacent collagen fibrils.