BISPHOSPHONATE MODULATES PROLIFERATION AND DIFFERENTIATION OF RAT PERIODONTAL-LIGAMENT CELLS DURING WOUND-HEALING

Background: Periodontal ligament (PL) width is precisely maintained th roughout the lifetime of adult mammals, but the biological mechanisms that regulate the spatial locations of the cell populations for bone, cementum, and PL are unknown. Methods: As bisphosphonates induce ankyl osis in mouse molar teeth, we used ethane-1-hydroxy-1, 1-bisphosphonat e-(HEBP, Etidronate; Didronel) in combination with a periodontal windo w wound model to identify those cell populations involved in the regul ation of PL width during the reformation of cellular domains after wou nding. Four groups of Wistar rats were wounded by drilling through the alveolar bone and extirpation of the PL. Rats were administered HEBP for 1 week and then sacrificed or allowed to recover for an additional week prior to sacrifice. Control rats were sacrificed after 1 or 2 we eks. One hour prior to sacrifice, rats were injected with H-3-thymidin e to label proliferating cells. Tissue sections were immunohistochemic ally stained for osteopontin (OPN) or bone sialoprotein (BSP) or were prepared for in situ hybridization (BSP) to identify extra- and intrac ellular expression of these non-collagenous bone proteins associated w ith periodontal healing. Results: HEBP treatment for 1 week induced a twofold increase in the thickness of the alveolar bone matrix in which weak immune-staining for OPN and BSP mRNA signal was seen. During the recovery phase the increased bone width was reduced but was still con siderably thicker than in control (P < 0.001). OPN staining as well as the BSP mRNA signal were much more intense than at 1 week. HEBP induc ed a > 40% reduction of PL width which returned to normal dimensions f ollowing the recovery phase. HEBP also modulated PL cell proliferation and differentiation: PL cell counts and labelling indices were reduce d fivefold after 1 week of HEBP but returned to control values after t he recovery phase. In controls, PL cells did not express OPN and BSP, but after HEBP treatment, and particularly after the recovery phase, P L cells expressed both of these markers intensely. In contrast, gingiv al and pulp connective tissues that were contiguous with the PL were n ot stained for OPN and did not express BSP mRNA after HEBP treatment. Conclusions: While wounding induced transient increases of proliferati on which were followed by repopulation of the extirpated tissue, the e ffects of HEBP on cell differentiation were independent of wounding. H EBP modulates the differentiation of PL cells and recruits cells that contribute to alveolar bone formation and loss of PL width homeostasis . Conceivably, bisphosphonates could be used therapeutically to select ively alter the differentiation of PL cells and promote the formation of alveolar bone and cementum. (C) 1997 Wiley-Liss, Inc.