Aspects ultrastructuraux du mecanisme de la calcification

Abstract

1. 1. The mechanism of calcification of mandibular bone, dentin, and enamel was studied in human foetal material with the electron microscope and electron diffraction. 2. 2. Preceded by the elaboration of an organic matrix in extra-cellular location, calcification of osseous tissue begins along the collagen fibrils, then extends rapidly to the interfibrillar ground substance. 3. 3. The organic matrix of dentin, which differentiates around the odontoblastic processes, undergoes a striking modification prior to calcification. 4. 4. The calcification of enamel starts very soon after the extracellular elaboration of matrix. Protein fibres in the matrix are difficult to demonstrate before the appearance of apatite crystals, but, by gentle decalcification of early mineralizing enamel on electron microscope grids, a fibrillar structure can be seen within the inorganic crystals. 5. 5. From the ultrastructural point of view, calcification appears to be a progressive phenomenon, beginning with the formation of punctiform germs of inorganic crystallization and continuing by growth in size of apatite crystals. The orientation and the nature of the fibrous proteins are significant factors, since they regulate the position and form of the inorganic crystals. But the mechanism relating to the initiation of calcification must be sought among the physico-chemical modifications occurring in the amorphous phase of the ground substance.