Acidic lipids associated with the local mechanism of calcificaiton: a review.

Two current areas of research on hard tissues have focused upon acidic lipids associated with the local mechanism of calcification and the presence of matrix vesicles as the loci for initial mineralization. Data which show that acidic phospholipids are a component of the vesicle membrane provide a common denominator between these two areas. Furthermore, studies on vertebrate, microbial and synthetic lipoprotein calcification indicate that acidic phospholipids play a pivotal role in the local mechanism. Through a sequence of initial Ca2++ binding followed by desolvation and ion concentration, the acidic phospholipid rich membrane provides an environment in which calcification can be facilitated. A proposed mechanism includes in additon to Ca2++ binding, the formation of (CaHPO4)2 dimers and their condensation into a Ca9 (PO4)6 unit bound to the membrane. The bound unit functions as a nucleus for the formation of additonal units to form an amorphous calcium phosphate cluster. Conversion to crystalline apatite would require dehydration of the environment, either by mineral build up exceeding the hydrophobic domain or by breakdown of the membrane.