Relationships between tooth eruption, occlusion and alveolar bone resorption: histochemical and cytological studies of bone remodeling on rat incisor alveolar bone facing the enamel after root resection.

The labial side of rat incisor alveolar bone facing the enamel is continuously resorbed as the result of compressive force produced by the occlusion and eruption of incisors. In order to clarify the relationship between this mechanical compressive force and the bone cells involved in bone remodeling, we examined morphological changes occurring in the alveolar bone once the compressive force was eliminated by the removal of the proliferative odontogenic base (root resection according to Berkovitz and Thomas, 1969). After root resection, the incisor migrated halfway along the socket. On the crestal part where incisor still existed, active osteoclasts were prominent on the bone surface, and flattened mononuclear cells lay close upon active osteoclasts. Sinusoidal blood vessels or capillaries were observed at short distances from the bone surface. On the basal part where socket was vacant, osteoblasts lined up on the newly formed bone, and the osteogenic cell layer lay on the osteoblasts. Between the two parts, which correspond to the reversal phase proposed by Baron (1977), osteoblastic cells with developed cell organelles increased in number and the distance between blood vessels and bone surface increased. Osteoclasts reduced their activities, and osteoblastic cells often wedged themselves between the osteoclasts and bone surface. These findings indicate that the elimination of compressive force mediated by incisors leads to the activation of osteoblastic cells and inactivation of osteoclasts, which results in a conversion from bone resorption to bone formation. Thus, osteoblastic cells may play an important role in controlling osteoclastic activity in conversion from bone resorption to bone formation, partly by a direct effect and partly by controlling the access of blood vessels to the bone surface.