The basic and applied biology of tooth eruption

The dentition and the alveolar process of each jaw develop simultaneously so that, by the time the crown is completed and eruption begins, the crown is enclosed in a crypt within alveolar bone. Thus, the eruption of a tooth to its functional position involves discretely localized, bilaterally symmetrical bone resorption to produce an eruption pathway and bone formation to fill in the space previously occupied by the crown and growing roots. Studies of crypt surfaces during eruption confirm this polarization of alveolar bone metabolism around a tooth with respect to both bone tells and mineralized surface topography. Experimental studies of tooth eruption have shown that the dental follicle, the dense connective tissue investment of the tooth, is necessary for eruption and that neither bone resorption nor bone formation occur without the adjacent part of the dental follicle. Early in eruption the coronal part of the follicle accumulates mononuclear cells which have cytochemical and ultrastructural features of osteoclasts and the apical part of the follicle, a site of intense cell proliferation, binds epidermal growth factor (EGF). The dental follicle contains a variety of proteins and the concentration of several change during eruption. Prominent among them are a reduction in matrix metalloproteinases and an increase in protoglycans as eruption proceeds. The contribution of these changes to those in cell proliferation, migration and differentiation during tooth eruption present experimental opportunities for developmental biologists. The rate-limiting factor of the earliest (intraosseous) stage of tooth eruption is bone resorption and eruption can be accelerated or retarded by the local delivery of factors which increase or decrease the activity of osteoclasts. The later (supraosseous) phase of eruption proceeds faster and is limited primarily by root elongation and bone formation. For most teeth the development of a functional periodontal ligament is a late acquisition. Once a tooth pierces the oral epithelium, the reduced enamel epithelium and oral epithelium combine to form a tight cuff, the junctional epithelium, on the enamel surface. The apical migration and renewal of the junctional epithelium then keep pace with tooth movement while the tooth reaches its functional position. Alveolar bone resorption and formation, root growth and formation and renewal of the junctional epithelium are significant components of tooth eruption. Understanding their regulation and timing will lead to the rational, predictable clinical management of ectopic eruptions, tooth movement, alveolar atrophy, periodontal regeneration and site-directed placement of a stable junctional epithelium.