Cellular, molecular, and genetic determinants of tooth eruption.

Tooth eruption is a complex and tightly regulated process that involves cells of the tooth organ and the surrounding alveolus. Mononuclear cells (osteoclast precursors) must be recruited into the dental follicle prior to the onset of eruption. These cells, in turn, fuse to form osteoclasts that resorb alveolar bone, forming an eruption pathway for the tooth to exit its bony crypt. Some of the molecules possibly involved in the signaling cascades of eruption have been proposed in studies from null mice, osteopetrotic rodents, injections of putative eruption molecules, and cultured dental follicle cells. In particular, recruitment of the mononuclear cells to the follicle may require colony-stimulating factor-one (CSF-1) and/or monocyte chemotactic protein-1 (MCP-1). Osteoclastogenesis is needed for the bone resorption and may involve inhibition of osteoprotegerin transcription and synthesis in the follicle, as well as enhancement of receptor activator of NF kappa B ligand (RANKL), in the adjacent alveolar bone and/or in the follicle. Paracrine signaling by parathyroid-hormone-related protein and interleukin -1 alpha, produced in the stellate reticulum adjacent to the follicle, may also play a role in regulating eruption. Osteoblasts might also influence the process of eruption, the most important physiologic role likely being at the eruptive site, in the formation of osteoclasts through signaling via the RANKL/OPG pathway. Evidence thus far supports a role for an osteoblast-specific transcription factor, Cbfa1 (Runx2), in molecular events that regulate tooth eruption. Cbfa1 is also expressed at high levels by the dental follicle cells. This review concludes with a discussion of the several human conditions that result in a failure of or delay in tooth eruption.