Ectopic mineralization in fibroblast cultures.

A human gingival fibroblast cell line was cultured in a DM-153 medium supplemented with 10% bovine serum. The cells in culture showed both intracellular and extracellular ectopic mineralization. Ultrastructurally, mineralization began within the round and irregularly shaped vesicles contained in large cytoplasmic vacuoles of fibroblasts. The first identifiable needle-shaped crystal was deposited on a point of the inner leaflet of the vesicle membrane. With the increase in size of the crystals, the membranous envelopes were gradually lost, and lattice planes (8.2 A) occurred within the crystal thickness. Crystals radiating from each mineralization center formed mineralized spherules in cytoplasmic vacuoles. The crystal clusters were extruded from the cells through membrane fusion or cellular degeneration in long-term cultures. These liberated clusters formed extracellular mineralizing matrices around the fibroblasts. In extracellular mineral deposition, the initial crystals arose within extracellular matrix vesicles, but were not associated with collagen, elastic fibers, and any other structures. These findings indicate that mineral accumulation and phase transformation of amorphous mineral to a crystalline form take place within the vesicular structures. This provides additional evidence for the view that the initiation of calcification and mineralization requires a microenvironment limited by a membranous (vesicular) structure derived from cells.