An electron microscopic observation on cells found in bone resorption area incident to experimental tooth movement

For the purpose of investigating many of the cells in the bone resorption area incident to experimental tooth movement and characterizing the role of these cells for the periodontal tissue resorption at the ultrastructural level, orthodontic force was applied to 24 male rats. Orthodontic elastics were inserted into the interproximal space of the upper first and second molars. The mesial side of the interradicular septum of the second molar was observed. The electron micrsocopic findings were as follows: 1. Cell cohorts were found in the undermining bone resorption area. They are mainly composed of fibroblasts, endothelial cells, undifferentiated cells, macrophages and several kinds of osteoclasts. 2. Four different kinds of osteoclasts could be recognized from the morphological features: a) small osteoclasts situated apart from the bone surface, b) large osteoclasts rich in rough ER, c) classic large osteoclasts and d) degenerating osteoclasts. 3. Two different types of undifferentiated cells were observed in these area. One was a spindle-shaped bright cell rich in rough ER and the other was a round-shaped dark cell with numerous mitochondria and free ribosomes. 4. Macrophages showing phagocytosis were also found in the bone resorption area.