Segregated localization of immunocompetent cells and osteoclasts in the periodontal ligament of the rat molar.

The spatial distribution of dendritic cells, macrophages, and their respective precursor cells in the periodontal ligament of rat molars was examined by means of ACPase enzyme histochemistry and immunohistochemistry. Intense reactions for ACPase were localized in both the multinucleated-and mononucleated cells of the periodontal ligament located exclusively in the portions of physiological bone resorption due to the physiological migration of the molar teeth. Immunohistochemical staining with OX6-monoclonal antibody that recognizes antigen-presenting cells such as dendritic cells and macrophages revealed the localization of immunopositive cells predominantly in the portions of the periodontal ligament that showed only trace reactions for ACPase. On the other hand, a large number of ED1-immunopositive cells, comprising a broad spectrum of cells of monocyte origin including dendritic cells and osteoclasts, displayed an almost even distribution throughout the periodontal ligament. Our current study is the first to show clear-cut in vivo morphological evidence that the cells of the bone-resorbing, osteoclastic cell lineage and those of the non-bone resorbing, macrophagic and/or dendritic cell lineages are exclusively localized in roughly the distal and proximal regions of the periodontal ligament of rat molars, respectively. An advantage is proposed for the use of the rat molar periodontal ligament as an in vivo model system for pursuing differentiation pathways of cells of the monocyte lineage, particularly of the osteoclastic cells.