Stem cell properties of Gli1-positive cells in the periodontal ligament.

BACKGROUND: The periodontal ligament (PDL), which surrounds the tooth root, contains mesenchymal stem cells (MSCs) capable of differentiating into osteoblasts, cementoblasts, and fibroblasts under normal conditions. These MSCs are thought to have important roles in the repair and regeneration of injured periodontal tissues. However, since there is no useful marker for MSCs in the PDL, the characteristics and distributions of these cells remain unclear. Gli1, an essential hedgehog signaling transcription factor, functions in undifferentiated cells during embryogenesis. Previous studies have demonstrated that the dental epithelial and mesenchymal cells positive for Gli1 in developing teeth have stem cell properties, including the ability to form colonies and pluripotency. Therefore, the focus of this review is the stem cell properties of Gli1-positive cells in the PDL, with an emphasis on the differentiation ability of osteoblasts for the regeneration of periodontal tissues. HIGHLIGHT: Lineage tracing analysis identified Gli1-positive PDL cells as MSCs that contribute to the formation of periodontal tissues and can regenerate alveolar bone. CONCLUSION: Gli1 is a potential stem cell marker in the PDL. A more definitive understanding of the functions of Gli1-positive cells could be useful for the development of regenerative methods using the MSCs in the PDL.

Sonic Hedgehog Signaling and Tooth Development.

Sonic hedgehog (Shh) is a secreted protein with important roles in mammalian embryogenesis. During tooth development, Shh is primarily expressed in the dental epithelium, from initiation to the root formation stages. A number of studies have analyzed the function of Shh signaling at different stages of tooth development and have revealed that Shh signaling regulates the formation of various tooth components, including enamel, dentin, cementum, and other soft tissues. In addition, dental mesenchymal cells positive for Gli1, a downstream transcription factor of Shh signaling, have been found to have stem cell properties, including multipotency and the ability to self-renew. Indeed, Gli1-positive cells in mature teeth appear to contribute to the regeneration of dental pulp and periodontal tissues. In this review, we provide an overview of recent advances related to the role of Shh signaling in tooth development, as well as the contribution of this pathway to tooth homeostasis and regeneration.

Sonic Hedgehog Regulates Bone Fracture Healing.

Bone fracture healing involves the combination of intramembranous and endochondral ossification. It is known that Indian hedgehog (Ihh) promotes chondrogenesis during fracture healing. Meanwhile, Sonic hedgehog (Shh), which is involved in ontogeny, has been reported to be involved in fracture healing, but the details had not been clarified. In this study, we demonstrated that Shh participated in fracture healing. Six-week-old Sprague-Dawley rats and Gli-CreERT2; tdTomato mice were used in this study. The right rib bones of experimental animals were fractured. The localization of Shh and Gli1 during fracture healing was examined. The localization of Gli1 progeny cells and osterix (Osx)-positive cells was similar during fracture healing. Runt-related transcription factor 2 (Runx2) and Osx, both of which are osteoblast markers, were observed on the surface of the new bone matrix and chondrocytes on day seven after fracture. Shh and Gli1 were co-localized with Runx2 and Osx. These findings suggest that Shh is involved in intramembranous and endochondral ossification during fracture healing.

Boric acid inhibits alveolar bone loss in rat experimental periodontitis through diminished bone resorption and enhanced osteoblast formation.

BACKGROUND/PURPOSE: Inhibition of bone resorption is essential for periodontal treatment. Recently, it has been suggested that boric acid suppresses periodontitis, but the mechanism of this inhibition is still not well understood. Therefore, to analyze the cellular response to boric acid administration, we histologically evaluated alveolar bone in experimental periodontitis of rats administered boric acid. MATERIALS AND METHODS: 5-0 silk ligatures were placed around the cervix of the second maxillary molars of 4 week-old rats treated with or without boric acid. Five and 14 days after ligature placement, the periodontal tissues between first and second molars were investigated histologically and immunohistochemically using antibodies to CD68, cathepsin K, and α-smooth muscle actin (SMA). RESULTS: Five days after the beginning of the experiment, many CD68-positive cells appeared in the periodontal tissues with ligature placement without boric acid administration. Also, the number of cathepsin K-positive osteoclasts had increased on the surface of alveolar bone. However, boric acid administration prevented severe bone resorption and reduced the number of cells positive for CD68 and cathepsin K. At day 14 post treatment, cells positive for α-SMA were seen in the periodontal tissues after boric acid administration, whereas no such cells were found around the alveolar bone without the administration of boric acid. CONCLUSION: Boric acid inhibited the inflammation of ligature-induced periodontitis. This agent might reduce bone resorption by inhibiting osteoclastogenesis and also could accelerate osteoblastogenesis.

Effects of tooth storage media on periodontal ligament preservation.

BACKGROUND/AIMS: An easily available tooth storage medium is required to preserve a tooth after avulsion. Milk and Hank's balanced salt solution (HBSS) are recommended as tooth storage media, and egg white is also reported to be comparable with milk. The aim of this study was to histologically and immunohistochemically evaluate the effect of different tooth storage media on the periodontal ligament (PDL) of extracted teeth. MATERIALS AND METHODS: This experiment used HBSS, milk, and egg white as tooth storage media. A total of ninety-six 6-week-old male Sprague-Dawley rats were used in these experiments. In each experiment, six rats were used for each medium and for the control group. Extracted rat molar teeth were immersed in these three different storage media for 1 hour. In each medium, six samples (n=18) were fixed immediately, and the remaining samples (n=54) were subcutaneously transplanted. In the control group (n=24), the extracted teeth were fixed or transplanted immediately after extraction. At day 4, 1 and 2 weeks after transplantation, the teeth were examined by radiographic, histological, and immunohistochemical methods. The number of PDL cells in the storage media was also counted. RESULTS: Teeth immersed for 1 hour in milk showed the thinnest PDL. Immunohistochemistry of periostin and CD68 labeling suggested degradation of the extracellular matrix in the PDL. In the media used for immersion, more PDL cells were observed in milk than in the other solutions. After transplantation, the HBSS and egg white groups maintained adequate thickness of PDL but in the milk group, thinner PDL and ankylosis were observed. CONCLUSION: Adequate thickness of PDL was maintained in the egg white group, whereas the milk group showed disturbance in the PDL, which may lead to ankylosis.