The apical region of developing tooth root constitutes a complex and maintains the ability to generate root and periodontium-like tissues

Mammalian tooth root development is a long-term process during which root elongates along the apical direction and is accompanied with the formation of periodontium. Considering the heterogeneous apical region of developing root as a functional entity, we observed and characterized the developing apical complex of rat molar root. The aim of the present study was to investigate the characteristics and developmental capability of developing apical complex in situ and in vitro. Histological analysis of rat developing apical complex was performed using hematoxylin and eosin staining and immunohistochemical staining. Cell counting, bromodeoxyuridine incorporation, flow cytometry assays and western blot analyses were performed to assess the proliferation potential of developing apical complex cells in vitro, and its mineralization potential was investigated by alkaline phosphatase activity, alizarin red staining and reverse transcription-polymerase chain reaction analysis. In vivo transplantation of both developing apical complex tissues and cells were used to characterize the differentiation capacity of developing apical complex cells. Dental pulp cells were used as a control in this study. Isolated developing apical complex maintained the developmental capability to form tooth root/periodontal complex ectopically. Developing apical complex cells exhibited relatively higher proliferation and mineralization potential compared with dental pulp cells in culture. When cultured developing apical complex cells were putatively depleted of Hertwig's epithelial root sheath cells, only osteodentin-like tissues and fibrous connective tissues formed in cell-scaffold explants. The sustainable development ability of developing apical complex qualifies it as the growth center of tooth root and as a promising candidate source of cells for tooth root and periodontal regeneration.