The rat labial incisor alveolar bone facing the enamel and bearing the occlusal force was examined by electron microscopy after being compared with the lingual alveolar bone by histological and scanning electron microscopic (SEM) observations.
On the labial side, shallow resorptive lacunae were recognized all over the bone surface; these were mainly covered by osteoclasts and some mononuclear cells. The cement line was absent from the bone matrix. On the lingual side, residues of Sharpey's fibers, the bone formation surface and deep resorptive lacunae were observed by SEM. Histologically, bone remodeling areas showing both osteoclasts and active bone-forming osteoblasts on the bone surface, as well as many cement lines in bone matrix, were recognized.
Furthermore, electron microscopic and cytochemical studies demonstrated that mononuclear cells located close to osteoclasts displayed osteoblastic characteristics such as alkaline phosphatase activity, a developed Golgi apparatus, and a rough endoplasmic reticulum.
These findings indicate that continuous bone resorption occurs on the labial bone surface, while active bone remodeling occurs on the lingual surface. Even on the labial surface, osteoblastic cells close to osteoclasts seem to play an important role in the differentiation and/or activation of osteoclasts.