Healing of the rat tooth extraction socket occurs rapidly, indicating a mechanism for cancellous bone formation occurring swiftly throughout the matrix. The residual periodontal ligament is evident at 2 days after extraction and its rich collagen type III fibre content may form a template for future cancellous bone formation. In the remainder of the early tooth extraction socket, fibronectin staining was generalized. The widespread distribution of fibronectin staining has given rise to speculation that the function of fibronectin may be important in granulation tissue formation, by providing a template matrix for fibroblast migration. Osteoprogenitor cells migrated into the socket from the surrounding bone, and produced decorin and proMMP-13 (procollagenase-3). ProMMP-13 was only expressed at sites of new bone formation, e.g. the border of the recently formed trabecular islands or the periphery of the closing socket. Collagen type I fibres were formed later, and were especially evident at 6 days after extraction. The pattern of distribution of both collagen type I and III fibres were similar as they passed from the bone margin towards the centre of the socket - in the same direction as the forming bone trabeculae. Bone formation occurs by rapid movement of the osteoprogenitor cells along these collagen fibres to allow a rapid healing, rather than that of resorption followed by slow bone deposition.