Granule proteins of the dental follicle and stellate reticulum inhibit tooth eruption and eyelid opening in postnatal rats

Mechanisms of tooth eruption and orthodontic tooth movement

Teeth move through alveolar bone, whether through the normal process of tooth eruption or by strains generated by orthodontic appliances. Both eruption and orthodontics accomplish this feat through similar fundamental biological processes, osteoclastogenesis and osteogenesis, but there are differences that make their mechanisms unique. A better appreciation of the molecular and cellular events that regulate osteoclastogenesis and osteogenesis in eruption and orthodontics is not only central to our understanding of how these processes occur, but also is needed for ultimate development of the means to control them. Possible future studies in these areas are also discussed, with particular emphasis on translation of fundamental knowledge to improve dental treatments.

Effects of local injections of vasoactive drugs on eruption rate of incisor teeth in anaesthetized rats

OBJECTIVES

Previous studies have suggested that the vasculature in the dental pulp and periodontal tissues plays an important role in producing the eruptive force in continuously erupting incisors. The aim of the present study was to investigate the effects of local injections of vasoactive drugs on regional blood flow within the socket in association with axial tooth movements to clarify the role of the local vascular system on tooth eruption.

DESIGN

Twenty-two male Wistar rats, weighing 309+/-21 g (S.D.), were immobilized with halothane anaesthesia. We measured the regional blood flow within the socket using a laser Doppler flowmeter, and the axial movements of the mandibular incisor using a displacement detector. The local injections of the vasoactive drugs, adrenaline (0.001, 0.01, and 0.1 microg/kg body weight) and acetylcholine (0.05, 0.5, and 5 microg/kg), into the base of the incisor were performed by a microinjector at a rate of 1 microl/kg body weight.

RESULTS

The injections of various doses of adrenaline decreased the mean regional blood flow and eruption rate dose-dependently, while those of acetylcholine increased the mean regional blood flow and eruption rate dose-dependently. The changes in the regional blood flow and eruption rate were transient. Significant correlations (p<0.001) were obtained between the maximum and minimum values in the regional blood flow and in the eruption rate following injections of various doses of adrenaline and acetylcholine.

CONCLUSION

These results support the hypothesis that the eruptive force of the rat incisor is closely related to the vasculature within the socket.

Epidermal growth factor receptor distribution in pericoronal follicles: relationship with the origin of odontogenic cysts and tumors

OBJECTIVE

Investigate the distribution of epidermal growth factor receptor (EGFR) in pericoronal follicles as a predictor of progression to odontogenic cysts and tumors.

STUDY DESIGN

Immunohistochemical EGFR staining patterns (membrane-only, cytoplasm-only, or combined membrane and cytoplasmic staining) in the reduced enamel epithelium and nests of odontogenic epithelium associated with follicles of impacted molar teeth were evaluated. The staining pattern of 20 specimens of pericoronal follicle was compared with that of 16 normal oral mucosa samples and to squamous cell carcinoma samples.

RESULTS

Combined membrane and cytoplasmic staining was observed for normal oral mucosa mostly in proliferating layers (basal and suprabasal), decreasing in intensity toward the surface. Seven epithelial nests presented membrane-only staining, and the majority presented either a cytoplasm-only or a combined staining pattern. The staining patterns observed in reduced enamel epithelium were cytoplasm-only and combined.

CONCLUSION

EGFR membrane-only staining may be an indicator of increased potential for epithelial nests to become odontogenic cysts or tumors.

Injections of osteoprotegerin and PMA delay tooth eruption

Tooth eruption requires alveolar bone resorption that is regulated by the dental follicle. This is reflected by the fact that failures of eruption often can be traced to either osteoclast deficiencies or to dental follicle abnormalities. To achieve maximal osteoclastogenesis and subsequent alveolar bone resorption for eruption, we have hypothesized that a reduction in gene expression of osteoprotegerin (OPG) in the follicle of the first mandibular molar of the rat at Day 3 is needed. To determine if OPG affects eruption, postnatal rats were injected with varying concentrations of OPG from Days 1-9 postnatally. Such studies indicated that the eruption time of the first mandibular molar was significantly delayed by 1 day or more as a result of OPG injection. Injection of phorbolmyristate acetate (PMA), an activator of protein kinase C (PKC) that in turn upregulates OPG expression, also delayed eruption by 1 day. PMA was only injected from Days 1-4 such that PKC-alpha would be increased and activated. Previous studies had shown that PKC-alpha gene expression is downregulated at the time (Day 3) that OPG expression is downregulated. In this study, using reverse transcription polymerase chain reaction techniques to examine OPG gene expression showed that PMA injection increased OPG gene expression in the dental follicle at Day 3 as compared to the controls. Thus, either injecting OPG or enhancing its expression in the follicle at Day 3 by injecting PMA delays the time of tooth eruption. Consequently, regulation of OPG production by the dental follicle likely affects the alveolar bone resorption needed for tooth eruption.

The effect of partial damage to the enamel-related periodontium combined with root resection on eruption of the rat incisor eruption

Previous work has indicated that the enamel-related periodontium (ERP) has a role in the eruptive process of the rat lower incisor. By combining partial damage of this tissue with resection of the odontogenic organ, we examined the effect of the damage on subsequent incisor eruption. The connective tissue of the enamel-related periodontium was regenerated in less than 2 weeks, showing morphology close to normal. The injured part of the enamel organ was neither regenerated nor repaired, and a cement-like tissue, continuous with the true acellular cement, was formed on the denuded enamel. Before tooth exfoliation, the operated teeth erupted at a slower rate compared with root-resected and sham-operated incisors, probably because of the absence of a substantial part of the enamel organ due to surgical damage. As with the coronal dental follicle and the enamel organ in rat molars, the enamel-related periodontium and the enamel organ of rat incisors may have some control on their eruptive process.

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.

Effects of dexamethasone on tooth eruption in rats: differences in incisor and molar eruption

A requirement for tooth eruption is the resorption of alveolar bone. Because bone resorption is stimulated by dexamethasone both in vivo and in vitro, dexamethasone 21-phosphate, a soluble form of dexamethasone, was injected into rats to determine its effect on tooth eruption. Such dexamethasone injections accelerate the time of intra-osseous eruption in rat incisors but do not accelerate the eruption time of rat molars when injected into rats. The injections of dexamethasone 21-phosphate also accelerate the time of eyelid opening in the postnatal rats, as well as retarding growth, as measured by body weight. These effects of dexamethasone 21-phosphate parallel the effects of epidermal growth factor injections, including the absence of an effect on molar eruption. This suggests that the molecular signals for the initiation of tooth eruption (i.e., onset of bone resorption) differ between rat incisors and molars. Given that rat incisors are teeth of continuous eruption whereas rat molars are teeth of limited eruption, as are human teeth, care must be taken in extrapolating results derived from rat incisors to human dentition. In vitro, dexamethasone has no effect on the gene expression of either osteoprotegerin or epidermal growth factor in dental follicle cells derived from molars. Because osteoprotegerin expression during normal tooth eruption is transitorily inhibited early postnatally in the molar dental follicle to allow osteoclast formation, the absence of inhibition of its expression by dexamethasone could explain why dexamethasone does not accelerate eruption in molars.

Inhibition of tooth eruption in the rat by a bisphosphonate

Studies of osteopetrotic rodents suggest that localized alveolar bone resorption must occur if the tooth is to erupt. To test this hypothesis directly, we injected postnatal rats with pamidronate, a bisphosphonate that reduces bone resorption by osteoclasts. The results of these experiments demonstrate that this bisphosphonate inhibits the time of tooth eruption of both rat molars and incisors. Pamidronate does not inhibit the gene expression of the putative tooth eruption molecules, colony-stimulating factor-1 and c-fos, both of which are expressed in the dental follicle, the tissue that is required for eruption to occur. Pamidronate does increase the size of the osteoclasts, including an increase in the number of nuclei, suggesting that the precursor mononuclear cells can still fuse to form osteoclasts despite the reduced ability of the osteoclasts to resorb bone. Thus, we report the discovery of an agent that inhibits tooth eruption and also show that tooth eruption requires alveolar bone resorption.

Tooth eruption: theories and facts

The mechanisms of tooth eruption (i.e., the answer to the question of how and why teeth erupt) has been a matter of long historical debate. This review focuses on human and other mammalian teeth with a time- and spacewise limited period of eruption and analyzes recent observations and experimental data on dogs, rats, primates, and humans in a framework of basic biological parameters to formulate a guiding theory of tooth eruption. Acknowledging basic parameters (i.e., that teeth move in three-dimensional space, erupt with varying speed, and arrive at a functional position that in inheritable) eliminates a number of previously held theories and favors those that accommodate basic parameters, such as alveolar bone remodeling in association with root elongation, with possible correction factors in the form of cementum apposition and periodontal ligament formation. We have critically analyzed, summarized, and integrated recent findings associated with preeruptive movements of developing teeth, the intraosseous stage of premolar eruption in dogs, molar eruption in rodents, and premolar and molar eruption in primates. The variable speeds of eruption are particularly important. We conclude with basic principles of tooth eruption--that is, the type of signals generated by the dental follicle proper, the conditions under which teeth are moved and the clinical understanding to be derived from this knowledge.

The effect of removing the true dental follicle on premolar eruption in the dog

Eruption is a highly localized process during which the bone resorption and formation that occur on opposite sides of the tooth are dependent upon the surrounding soft tissues, the true dental follicle externally and the enamel organ internally. To examine the ability of the enamel organ to cause eruption the external layer (dental follicle) was removed just prior to and up to 4 weeks before eruption in 13 mandibular premolars in dogs and eruption followed clinically, radiographically and histologically. None of the teeth without dental follicles erupted but three teeth from which the follicle was separated then replaced did erupt. These data indicate that the enamel organ without the dental follicle cannot support tooth eruption and provide indirect evidence for the central role of the dental follicle, alone or in combination with the enamel organ, in eruption.

Isolation of granule proteins from cells of the dental follicle and stellate reticulum of rat mandibular molars

The presence of a dental follicle is required for eruption of teeth of limited eruption but it is uncertain if any molecules indigenous to the follicle regulate this eruption. However, electron-dense granules of unknown composition and function are present in the fibroblasts of the dental follicle of rat molars, as well as the adjacent stellate reticulum, before and during tooth eruption. Here the granules have been isolated; two proteins, of 167 and 200 kDa, have been determined by biochemical and immunological methods to be major components of the granules.