Osteoclasts in the dental microenvironment: a delicate balance controls dental histogenesis.

The impact of osteoclast activity on dental development has been previously analyzed but in the context of severe osteopetrosis. The present study sought to investigate the effects of osteoclast hypofunction,present in Msx2 gene knockin mutant mice (Msx2-/-), and hyperfunction, in transgenic mice driving RANK over-expression in osteoclast precursors (RANK(Tg)), on tooth development. In Msx2-/- mice, moderate osteopetrosis was observed, occurring exclusively in the periodontal region. Microradiographical and histological analyses revealed an abnormal dental epithelium histogenesis that gave rise to odontogenic tumor-like structures. This led to impaired tooth eruption, especially of the third mandibular molars. In RANK(Tg) mice, root histogenesis showed site-specific upregulation of dental cell proliferation and differentiation rates. This culminated in roots with a reduced diameter and pulp size albeit of normal length. These two reverse experimental systems will enable the investigation of distinctive dental cell and osteoclast communication in normal growth and tumorigenesis.