Synthetic Analogue of Bone Mineral Formation

X-ray diffraction measurements on whole bone have been interpreted to show that mature bone mineral consists of about 65%-apatite with the remaining calcium phos-phate present as a phase amorphous to X-ray diffraction. ' In synthetic systems, the pre-cipitation of hydroxyapatite (HA) in highly supersaturated solutions proceeds through a precursor amorphous calcium phosphate (ACP) phase which, unless stabilized, con-verts autocatalytically to HA.2 The possi-bility that the amorphous portion of bone mineral resembles synthetic ACP has been considered.3 A recent study showed that in avian bone resorption during the egg-laying cycle, the bone richer in amorphous is re-sorbed preferentially and thus is metaboli-cally more active than the crystal-rich bone.4 Whatever the nature of that portion of bone mineral which is amorphous to X-ray diffrac-tion, it is clear that it is biologically impor-tant. Thus, the formation of ACP and the nature of its transformation to HA have been investigated to aid in understanding the atomic structure of bone and the mech-anism of bone calcification. This paper discusses recent results from this laboratory, published and unpublished, on the nature of ACP and HA and the trans-formation of ACP to HA. Emphasis is placed on the relevancy of these in vitro studies to understanding bone mineral. Preliminary results are given which suggest that the por-tion of bone mineral that is amorphous to X-ray diffraction is not identical to synthetic ACP. Evidence is presented to illustrate why we have modified somewhat our past views on the nature of bone mineral. The material