Thrombospondin promotes resorption by osteoclasts in vitro.

Dentine slices were soaked in TSP-1, FN or buffer before carrying out resorption assays using bone cell suspensions prepared from the long bones of 18-20-day old pre-hatch chick embryos. Soaking dentine slices in FN resulted in a five-fold increase in resorption over controls, while TSP-1 gave a two-fold increase. However, whereas the increase in resorption on FN-soaked slices could be explained by a vast increase in cell adhesion to the slices, TSP-1-soaked wafers showed no increase in cell adhesion. TSP-1, therefore, appeared to promote resorption by a mechanism other than simply increasing adhesion of stromal cells to the dentine slices. In confirmation of this, TSP-1 was able to further increase resorption on FN-soaked slices where cell adhesion was already maximal. We conclude that the effect of TSP-1 is on resorption, and that osteoclast resorption can be regulated by the presence of TSP in the matrix.