Formation of osteoclast-like cells by periodontal ligament and gingival fibroblasts

Objective: Several studies indicate that periodontal ligament fibroblasts (PLF) have some similarities to osteoblasts, e.g. PLF express high levels of alkaline phosphatase and they have the capacity to induce the formation of osteoclast-like cells. Whether a second population of tooth-associated fibroblasts, gingival fibroblasts (GF), has a similar capacity, is unknown. Methods: PLF and GF were isolated and cultured from extracted human permanent molars. For osteoclastogenesis experiments, PLF and GF were co-cultured with peripheral blood mononuclear cells (PBMCs) in the presence and absence of dexamethasone and 1á,25dihydroxycholecalciferol (dex+vit D3) on plastic and on cortical bone slices. After culturing the cells were stained for tartrate resistant acid phosphatase (TRACP). Results: TRACP-positive multinucleated cells (MNCs) were more abundant in co-cultures with PLF than in co-cultures with GF. Conditioned medium from GF but not from PLF inhibited MNC formation. Addition of dex+vit D3 to co-cultures resulted in both fibroblast cultures in higher numbers of MNCs. The MNCs were not capable to degrade bone in neither fibroblast-PBMCs co-culture, suggesting that both fibroblast populations secrete inhibitors of bone resorption or that the osteoclast-like cells were not functional. We next investigated whether expression of molecules important for osteoclastogenesis differed between the two types of fibroblasts and whether these molecules were regulated by dex+vit D3. OPG was detected at high levels (>300 pg/ml) in both fibroblast cultures, whereas RANKL could not be detected (< 16 pg/ml). Since RANKL was not detected in our system, we investigated whether formation of osteoclast-like cells could occur in the absence of RANKL. Equal numbers of MNCs were formed when PBMC were cultured with M-CSF compared to PBMCs cultured with M-CSF and RANKL. Bone resorption, however, was only present in cultures to which RANKL was added. Conclusion: PLF are better equipped to induce formation of osteoclast-like cells from precursors than GF.