Effect of chondroitin sulfate and biglycan on the crystallization of hydroxyapatite under physiological conditions

Crystallization of hydroxyapatite (HA) in collagen gels has been studied in vitro in order to mimic biomineralization under physiological conditions. In particular, the effect of chondroitine sulfate (CS) and biglycan (BG) additions to the gel on the growth kinetics and morphology of HA crystals has been investigated. Large molecules of CS disrupt the gel and lower its viscosity, thus increasing the nucleation and growth rates as well as the overall mass of crystallized HA clusters, while the individual crystallite shape and size (similar to 1 mu m long) remain unchanged. In contrast, the presence of about 100 mu g/ml gel of BG limits the crystal size to less than 100 nm, since the protein cores of BG bind to collagen fibers forming a three-dimensional fiber network with a "lattice" constant of about 50 nm. This network provides preferential sites for ordered heterogeneous nucleation and also prevents clustering of HA crystals. Besides the general importance of these results for studying the biomineralization processes, they may explain the apparition of osteoporosis in BG-gene deficient mice. (C) 1999 Elsevier Science B.V. All rights reserved.