The recent concept of periosteal distraction osteogenesis (PDO) is based on the potential of Mesenchymal Stem Cells (PMSCs) of the periosteum to differentiate into osteoblasts by mechanical stimulus of tension on the periosteum, to fill the space over the underlying bone. However, it also seems there is a role of both progenitor cells from the blood and osteoblasts from the underlying original bone in bone formation in this technique. Therefore the precise contribution of the Mesenchymal Stem Cells (MSCS) and the old bone to de novo bone formation needs to be clarified. The purpose of this article is to investigate the role of PMSCs in de novo bone formation in periosteal distraction in osteogenesis.