High-resolution electron microscopic study of the initial stage of collagen mineralization.

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The purpose of this study is to investigate the initial mineralization mechanism of collagen.Specimens were selected from examples of dentin containing unusual collagen agregates. Initial crystals (about 2nm thick) on normal collagen assumed needle or thin-platelet shape. On the basis of crystal morphology, initial crystals were thought to be made up of hydroxiapatite or octacalcium phosphate.But, since it was impossible to view crystal lattices and micro-area electron diffraction patterns clearly, I was unable to determine crystal type.Mature crystals (about 4nm thick) were platelet-like in shape.Their crystal-lattice spacing (about 0.82nm) indicated hydroxiapatite.Similar results were produced from crystals on unusual collagen agregates.Because unusual collagen agregates form neither periodic striations nor hole zone, their mineralization occurs independent of both these phenomena.Since the direction of the crystal c-axis coincides with that collagen-molecule long axis, it seems that crystal commence depositing lengthwise along collagen molecules.In conclusion, this study suggests that the initial mineralization of collagen comprises three events : 1) initial precipitation of octacalcium phosphate between collagen molecules, 2) subsequent transformation of octacalcium phosphate into hydroxiapatite, and 3) crystal growth of this hydroxiapatite.