Visualization of cellular dynamics associated with orthodontic tooth movement

Abstract

ORTHODONTIC tooth movement appears to be dependent upon the presence Of osteoblasts and osteoclasts within the periodontal ligament and adjacent bone.l-’ A force applied to a tooth creates areas of pressure and tension. In areas of pressure, bone resorption, in association with the appearance of osteoclasts, is stimulated. In areas of tension, osteoblasts arise and form osteoid, which is subsequently calcified. The bone remodeling process allows the tooth to move and thus maintain the physiologic width of the periodontal ligament.lm7 It has been presumed that osteoblasts and osteoclasts a,re derived from the proliferation and differentiation of undifferentiated mesenchymal cells, since increased mitotic activity of mesenchymal cells is observed in the supporting tissues of a tooth subjected to an orthodontic force.*, 3l 7 In fact, however, the histogenesis of these cells is in doubt. On the other hand, cellular proliferation can be documented by the use of radioactive precursors of DNA, because DNA is duplicated during cell division. The appearance of radioactivity within cell nuclei indicates the presence of “new” DNA which had been synthesized in an early phase of the process of cell division.8 Therefore, using a DNA precursor, tritiated thymidine, we investigated the role of cellular proliferation during orthodontic tooth movement, with emphasis on the formation of osteoclasts and osteoblasts.