A critical cellular event in tooth eruption is the formation of osteoclasts that are needed for bone resorption to form an eruption pathway. To analyze molecular regulation of osteoclast formation and activation, we examined the expression of osteoprotegerin (OPG), an inhibitor of osteoclast formation. In vivo, the gene expression of OPG is reduced in the dental follicle of the first mandibular molar of the rat at day 3 post-natally and in the mouse at day 5. This correlates with the days of maximal mononuclear cell influx and osteoclast numbers in the rat and mouse. Thus, inhibition of OPG gene expression on these days might allow osteoclasts to be formed and/or activated. In vitro studies demonstrated that both colony-stimulating factor-1 and parathyroid hormone-related protein reduced OPG gene expression in follicle cells, suggesting that these are candidate molecules for the in vivo inhibition of OPG expression. Osteoclast differentiation factor (ODF) immunolocalizes to the alveolar bone stromal cells adjacent to the follicle, whereby it might act to stimulate fusion of the mononuclear cells in the follicle.
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