Effects of fibronectin on hydroxyapatite formation

There is increasing evidence that noncollagenous matrix proteins initiate b one mineralization in vivo. Fibronectin, which is present during the early phases of mineralization, may contribute to this process in bone tissues. I n this context, the mineralization potential of fibronectin was tested in a n agarose gel precipitation system and a metastable calcium phosphate solut ion. The protein inhibited the precipitation of calcium phosphate crystals in solution but had no apparent effect in gel. Conversely, fibronectin stim ulated crystal formation when apatite powder was used to seed crystal growt h in gel. Although these results in vitro do not clearly indicate that fibr onectin is involved in the mineralization process, they are consistent with in vivo events. Free fibronectin (e.g. in biological fluids) could inhibit crystal growth but might also activate the mineralization process when abs orbed on apatite powder in a bone environment and areas of ectopic minerali zation. (C) 1999 Elsevier Science Inc. All rights reserved.