Effects of hydroxyapatite particles on periodontal ligament fibroblast-like cell behavior

Although hydroxyapatite (HA), a synthetic calcium phosphate, is used in restoring bone defects associated with periodontal diseases, its specific effect on the periodontal ligament fibroblast population during the regeneration process is unclear. To determine the cellular events occurring in the presence of HA, human periodontal ligament fibroblasts (HPLF) were isolated and maintained in culture. The specificity of the cells was evidenced by their morphology, deposition of extracellular matrix components, and alkaline phosphatase (ALP) activity (as a marker of osteoblastic differentiation of HPLF). Phase-contrast investigations revealed morphological alterations of cells in contact with HA particles. Transmission electron microscopy demonstrated the phagocytotic process of HPLF toward HA particles. Moreover, the presence of HA particles was significantly related to an increase in the protein synthesis activity and a decrease in the proliferation and ALP-specific activity of HPLF. These results provide new information on the phenotypic expression of HPLF, which is comparable to that of osteoblastic cells. A subpopulation of HPLF may be influenced by the presence of HA to undergo transient dedifferentiation prior to redifferentiating into osteoblasts. This process may be important as a means by which HA acts as an osteoconductive material. This experimental study improves our understanding of the cellular processes which occur during healing and regeneration of periodontal defects after implantation of biomaterials.