The function of citrate in bone: platelet adhesion and mineral nucleation.

The mineral component of bone consists of platelets of apatite 2-6 nm thick, which are bound together as stacks of up to 30 platelets located around and between the collagen fibrils. These coherent stacks of platelets, which contribute significantly to the compressive stiffness and strength of bone, are held together by glue. We show that this glue is most likely citrate, 2 wt% of bone. Nuclear magnetic resonance spectroscopy analyses have shown that citrate is attached to calcium atoms in bone, most likely connected to the faces of apatite crystals. Citrate molecules bound on apatite plates also act as the epitaxial site for the growth of additional apatite crystals, forming stacks of mineral plates. The presence of citrate should also contribute to the bonding of minerals to collagen and the resulting mechanical properties of bone. Such insights provide a deeper understanding of bone biology. These findings should also stimulate new studies on the mechanics of bone at the nanoscale aimed to characterize experimentally and computationally the strengths of these two types of interfaces, the mineral-collagen, and mineral-mineral interfaces in the presence of citrate, and their effect on bone's mechanical properties.