Ultrastructure of early calcification in cervical ossification of the posterior longitudinal ligament

A case of ossification of the cervical posterior longitudinal ligament was investigated with the electron microscope. The posterior longitudinal ligament was composed of bundles of collagen fibers intermingled with occasional fibroblasts and rare blood vessels. Some ligaments contained matrix vesicles in the vicinity of degenerated cells. Hydroxyapatite crystals were frequently precipitated within the matrix vesicles. These findings are similar to the fine structure of the early stage of calcification in normal and pathological calcifying tissues described previously. In this study, the calcification process of the posterior longitudinal ligament suggests that matrix vesicles originate from degenerated cells, and acquire hydroxyapatite crystal deposits. Some eventually coalesce to form a large calcifying mass. Substantial amounts of collagen fibers comprising the ligament may serve an important role in orienting apatite crystal precipitation.