Hydroxyapatite formation in collagen, gelatin, and agarose gels.

Collagen has long been suspected to be a promoter of hydroxyapatite (HA) deposition in bone. This theory was tested by comparison of HA formation in gels composed of collagen, gelatin or agarose. Collagen gels supported substantially more HA precipitation than gelatin gels, but slightly less than agarose gels. Analysis of the relative diffusion rates for calcium in these matrices indicated that, in this system, amount of HA formation is dependent upon the rate of diffusion. Under conditions in which the diffusion rates for collagen and agarose gels were comparable, similar amounts of HA were formed. This suggests that fibrillar collagen is not per se a promoter of HA deposition. Extracellular matrix macromolecules may influence calcification by restricting ionic diffusion through connective tissues.