Bone structure: from angstroms to microns.

Bone has a complex hierarchical structure, which despite much investigation, is still not well understood. Here we bring together pieces of this complicated puzzle, albeit from different sources, to present a tentative overview of bone structure. The basic building blocks are the extremely small plate-shaped crystals of carbonate apatite, just hundreds of angstroms long and wide and some 20-30 A thick. They are arranged in parallel layers within the collagenous framework. At the next hierarchical level these mineral-filled collagen fibrils are ordered into arrays in which the fibril axes and the crystal layers are all organized into a 3-dimensional structure that makes up a single layer or lamella of bone a few microns thick. The orientations of the collagen fibrils and the crystal layers in alternating lamellae of rat bone differ such that in the thinner lamellae, the fibrils and the crystal layers are parallel to the lamellar boundaries. In the thicker lamellae the fibrils are parallel to the boundary, but the crystal layers are rotated out of the plane of the boundary. In many bones these alternating lamellae are organized into even larger ordered structures to produce what is truly a remarkably ordered material, all the way from the molecular scale to the macroscopic product.