“…A possible explanation may be related to the colonization of the healing wound by periodontal progenitor cells, a prerequisite for the formation of new cementum, a new periodontal ligament apparatus, and new alveolar bone (6,9,18,21,46). A key factor for this process is the periosteum because it may act both as a source of osteocompetent cells and as a barrier against the infiltration of epithelial cells into the healing site (6,9).…”