The dynamics of osteon (Haversian system) calcification was investigated under the electron microscope applying a specially devised dissection technique in order to examine units in which the amount of calcium salts had been previously established.

After a sudden initial penetration of a large percentage of the matrix by elongated needle-shaped crystallites, the apatite is laid down slowly, starting from the broad bands at the major collagen periods. Here small spots corresponding to foci of crystal inception are distributed along the axis of the collagen and fuse in needle-shaped crystallites, reaching a maximum width of 40–50 Å. Successively the crystallites are long enough to span two or more major periods, completely obscuring the fiber structure. Of course as the osteon calcification increases, the areas covered by needle-shaped crystallites get larger.

The significance of these findings is discussed as well as the differences between calcification of the osteonic bone and calcification of some primary and coarse-fibered bone.