The ultrastructure of bone as revealed in electron microscopy of ion-milled sections

Mineral makes up more than half the volume of bone, but its spatial and structural relationship to collagen and other proteins is still a matter of debate. Due to the nanometer-size of bone crystals this matter can be resolved only with transmission electron microscope (TEM) images. Using sections cut with an ultramicrotome, previous investigators determined most mineral lies in the 40 nm wide gap zone in collagen fibrils. Using less invasive sectioning methods (ion milling and focused ion beam [FIB]) reveals that most mineral is extrafibrillar, occurring in the form of mineral lamellae, polycrystalline plates 300 nm or more long, packed around collagen fibrils in stacks of four or more lamellae <1 nm apart. While Ca and P also occur in the gap zone, they do not appear to be in the form of well-crystallized apatite. This new model for bone ultrastructure resolves outstanding problems presented by the previous model.