Regional differences of expression of bone morphogenetic protein-2 and RANKL in the rat dental follicle.

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.