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In the present study, the normal root formation of the molars and the tissue reactions to the infection or the subsequent endodontic procedures on the developing molars in the rats were studied histologically. The pulp cavity of the developing molars in the 3- week-old rats were exposed and allowed to remain open to the oral environment for a period of 1 week. The infected root canals were then covered with cement after the endodontic treatment. Microscopic examination of the teeth in the normal animals revealed that in the 3-week-old animals the root length of the lower first molars was about 1/2 of that of the matured teeth. The root of the lower first molars matured at 9 weeks after birth. After 1 week of pulp exposure, the epithelial sheath was still observed in the region of the root apex despite the necrosis of the pulp tissue. In the later stage, two types of periapical tissue reactions to the infection were observed. One was the destructive reaction in which the periapical inflammation progressed in intensity and severity. The other was the repairtive reaction in which an apical closure by the osteoid tissue and a cementum-like tissue were seen. When the infected root canals were covered with cement after the endodontic procedure, the periapical tissue showed also two types of reaction, i. e., one was the destructive and the other was the repairtive. In the latter type, however, continued root growth and closure of the foramina with hard dental tissue and osteoid tissue were much more progressive.

Histological observations on pulpal and periapical tissue reactions after pulp exposure of developing molars.