Objectives: Tooth eruption is a localized event that requires the expression of certain molecules at precise times to regulate osteoclastogenesis, and parathyroid hormone-related protein (PTHrP) and vascular endothelial growth factor (VEGF) may be two of those molecules. Although PTHrP is produced in the stellate reticulum (SR) of the tooth and exerts its effect on the adjacent dental follicle, its expression pattern in the SR is unknown. VEGF can substitute for CSF-1 to promote osteoclastogenesis and its expression is increased at days 9 to 11 in the rat dental follicle. Thus, it was the objectives of this study to determine the chronological expression of PTHrP, and to determine its effect on VEGF expression. Methods: Mandibles were surgically removed from new-born rats at days 1, 3, 5, 7, 9 and 11 and frozen in liquid nitrogen. Frozen sections (20 µm) of the first molar were cut, and fixed with ethanol. Laser capture microdissection (LCM) was then used to excise the SR, followed by isolation of RNA and subsequent RT-PCR to measure PTHrP gene expression. For VEGF analysis, dental follicle cells were incubated with PTHrP at concentration of 25 ng/ml for 0, 1, 2, 4, 6 and 8 hours. RNA was then isolated and VEGF gene expression was determined using real-time RT-PCR. Results: PTHrP was maximally expressed at day 7 postnatally. All of the other days except day 9 had significantly less expression than at day 7. In vitro, PTHrP upregulated VEGF120 and VEGF 164 expression after 4 hours of incubation with a maximum effect at 6 hours of incubation, the latter being significantly different from the untreated control. Conclusion: Because the secondary burst of osteoclastogenesis needed for eruption occurs around day 10, it is possible that PTHrP is stimulating this osteoclastogenesis by upregulating VEGF production. Sponsored by NIH Grant DE08911-14 to G.E.Wise.

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