Collagen fibrillar structure in mineralized and nonmineralized tissues

The four most important recent developments have been the following determinations: first, that collagen molecules have considerable flexibility so that in regions devoid of glycine—proline—proline or glycine—proline—hydroxyproline in each chain, substantial helix distortion can be allowed; second, that this flexibility is reflected by considerable azimuthal disorder in radially-packed fibrils, with the azimuthal order specified, principally, in the cross-linked overlap zones; third, that mineral crystal size and order are tightly linked to the perfection of the collagen packing, although in bone the crystal c-axis may be tilted by as much as ±20° from the fibril axis; and fourth, that the process of mineralization may cause rearrangement of the cross-linkages in the collagen overlap zones.