The dental follicle (DF) differentiates into the periodontal ligament. In addition, it may be the precursor of other cells of the periodontium, including osteoblasts and cementoblasts. We hypothesized that stem cells may be present in the DF and be capable of differentiating into cells of the periodontium. Stem cells were identified in the DF of the rat first mandibular molar by Hoechst staining, alkaline phosphatase staining, and expression of side-population stem cell markers. These cells were shown to be able to differentiate into osteoblasts/cementoblasts, adipocytes, and neurons. Treating the DF cell population with doxorubicin, followed by incubation in an adipogenesis medium, suggested that the adipocytes originated from stem cells. Thus, a possibly puripotent stem cell population is present in the rat DF.

The objectives of this anatomical study were to (1) determine if significant bone growth occurs in the base of the alveolar bony crypt of the first mandibular molar to move the tooth through the eruption pathway; (2) determine if the osteogenesis in the crypt correlates with the published chronological gene expression of bone morphogenetic protein-2 (BMP-2) in the dental follicle; and (3) determine chronologically and regionally the crypt bone activity. To accomplish this, the alveolar bony crypts of rat mandibular molars from postnatal days 3 to 18 were processed and examined by scanning electron microscopy (SEM). In addition, mandibles and teeth of ages 12-18 were prepared for light microscopy. SEM demonstrated that bone formation occurs in the basal (apical) portion of the alveolar bony crypt at day 3, whereas bone resorption concurrently is ongoing in the coronal region of the crypt. By day 9, the crypt is beginning to be reduced in depth as the result of basal bone formation, and by day 14, the base of the crypt immediately under the tooth is almost completely filled with bone to form the interradicular septum. At day 18, the tooth erupts as bone formation likely elevates the molar. Bone growth in the basal area of the crypt correlates with a previous study showing enhanced BMP-2 expression in the dental follicle. Thus, SEM indicates that the motive force of tooth eruption likely is bone formation at the base of the alveolar crypt and this osteogenesis may relate to BMP-2 production in the dental follicle.

The aim of this study was to determine the effect of interleukin-10 (IL-10) on the gene expression of osteoclastogenic regulatory molecules in rat dental follicle cells. Interleukin-10 is an anti-inflammatory cytokine that inhibits alveolar bone resorption, but the molecular basis for this is unknown. Alveolar bone resorption is required for tooth eruption and the dental follicle functions to regulate the osteoclastogenesis needed for eruption. It does this by regulating its expression of receptor activator of nuclear factor-kappa B ligand (RANKL), colony-stimulating factor-1 (CSF-1), and osteoprotegerin (OPG). In this study, dental follicle cells were treated with IL-10, and the effect on gene expression of CSF-1, RANKL, and OPG was measured by reverse transcription-polymerase chain reaction (RT-PCR). Interleukin-10 enhanced the expression of OPG and down-regulated the expression of RANKL and CSF-1. Laser capture microdissection was carried out to detect IL-10 gene expression in the dental follicle. Knockdown of the IL-10 gene expression in the follicle cells was accomplished using a short interfering RNA (siRNA) targeting IL-10 mRNA. In these knockdowns, RANKL expression was increased and OPG expression was decreased. All of these results suggest that IL-10 inhibits bone resorption by up-regulating OPG expression while down-regulating expression of RANKL and CSF-1.

Tooth eruption requires alveolar bone resorption and bone formation. The coronal half of the dental follicle probably mediates the bone resorption seen in the coronal region of the alveolar bony crypt, and the basal half of the follicle mediates bone growth in the basal region. We hypothesized that the expression of a gene for bone resorption--receptor activator of nuclear factor kappa B ligand (RANKL)--would be higher in the coronal than in the basal region of the follicle. Conversely, the level of expression of bone morphogenetic protein-2 (BMP-2), a gene for bone formation, would be higher in the basal region. Results obtained using laser-capture microdissection and real-time reverse transcription-polymerase chain reaction (RT-PCR) confirmed the hypothesis. Scanning electron micrographs of the bony crypt showed that the coronal area of the crypt was scalloped in appearance (bone resorption), whereas the basal area was trabecular (bone formation). Thus, the differences in bone activity at opposite poles of the crypt appear to be caused by differences in the regional expression of genes in the dental follicle and suggest a molecular mechanism whereby the dental follicle could regulate both the alveolar bone resorption and formation needed for eruption.

Tooth eruption in the rat requires bone resorption resulting from a major burst of osteoclastogenesis on postnatal day 3 and a minor burst of osteoclastogenesis on postnatal day 10 in the alveolar bone of the first mandibular molar. The dental follicle regulates the major burst on postnatal day 3 by down-regulating its osteoprotegerin (OPG) gene expression to enable osteoclastogenesis to occur. To determine the role of receptor activator of nuclear factor-kappa B ligand (RANKL) in tooth eruption, its gene expression was measured on postnatal days 1-11 in the dental follicle. The results show that RANKL expression was significantly elevated on postnatal days 9-11 in comparison to low expression levels at earlier time-points. As OPG expression is high at this latter time-point, this increase in RANKL expression would be needed for stimulating the minor burst of osteoclastogenesis. Tumor necrosis factor-alpha enhances RANKL gene expression in vitro and it may be responsible for up-regulating RANKL in vivo. Transforming growth factor-beta1 and interleukin-1alpha also enhance RANKL gene expression in vitro but probably have no effect in vivo because they are maximally expressed early. Bone morphogenetic protein-2 acts to down-regulate RANKL expression in vitro and, in vivo, may promote alveolar bone growth in the basal region of the tooth.