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Expression of the TGF-β and type I collagen genes during healing of extracted tooth sockets was studied using in situ hybridization and immunohistochemistry.1. Histological findingsA clot was formed in the extracted tooth socket 1 day after extraction. By 2days after extraction, fibrogenesis was evident in the clot. Bone trabeculae began to form from the basal wall of the tooth socket from 3days, and from the lateral walls from 3 to 5 days after extraction. The tooth sockets were filled by newly formed bone within 14 days after extraction.2. Gene expression of TGF-β 1At 1 day after extraction, the gene for TGF-β 1 was expressed in leukocytes near the clot, and in fibroblasts and osteoblasts in the remaining periodontal ligament after tooth extraction. The signals of osteoblasts associated with newly formed bone were increased from 3 days after extraction. Within 5 to 7 days after extraction, the most intense signals were observed in osteoblasts along the newly formed bone trabeculae and in fibroblasts. The signals in the cellular component at 7 days after extraction were stronger than those at 14 days after extraction.3. Gene expression of type I collagenThe gene for type I collagen was expressed more strongly than that of TGF-β 1. At 1 day, no signals were found in the cells of the clot, but fibroblasts and osteoblasts showed intense expression. After 2 days the distribution of signals was essentially the same as that for TGF-β 1.4. Immunohistochemistry of TGF-β 1At 1 day after extraction, immunolocalization of TGF-β 1 was observed in the clot, leukocytes, and in fibroblasts and osteoblasts in the remaining periodontal ligament after tooth extraction. Then, immunoreactions were observed in osteoblasts and fibroblasts. These results suggest that expression of the TGF-β gene regulates the synthesis of type I collagen by fibroblasts and osteoblasts, and plays a central role in cell proliferation and differentiation during the healing of extracted tooth sockets.