Comparative effects of parathyroid hormone on osteoblasts and cementoblasts.

Although bone, dentin and dental cementum are mesenchymal mineralized tissues composed mainly of collagen and hydroxy apatite, they differ markedly in their suceptibility to resorption. Bone undergoes physiological resorption to which the dental tissues appear to be resistant. Recently, the cells covering bone surfaces have been attributed a regulatory role in osteoclastic bone resorption by exposing the bone surface following stimulation with hormones and inflammatory mediators, thereby allowing osteoclasts to colonize the bone surface. In the present study, it was shown that reparative cementum-forming cells covering an experimental cavity in the root surface of replanted monkey incisors were unaffected by parathyroid hormone, a potent mediator of bone resorption. In parallel experiments, endocranial osteoblasts exposed bone surface by widening their inercellular spaces. It was concluded that the layer of cells covering the root surface forms a protective barrier against resorption which serves to preserve the integrity of the dental root.