**6. Endothelial progenitor cells (EPCs) seeded for bone tissue engineering**

EPCs are bone marrow-derived precursor cells and express CD34 molecules. They have the ability to differentiate into endothelial cells and ultimately contribute to the process of angiogenesis [35]. They have been reported to be resident cells in the peripheral blood and potentially contribute to the initiation of neovascularization [36]. There have been several studies demonstrating the use of EPCs in tissue engineering. Zigdon-Giladi and co-workers in their nude mouse model study with calvarial defect demonstrated that human EPCs could enhance the processes of vasculogenesis and osteogenesis [37]. They concluded that there was a significant increase in blood vessel density as well as increased extra-cortical bone height and length in the human EPC-transplanted group compared to the control. Furthermore, EPCs seeded on Gelfoam scaffold were reported to be efficient in stimulating cranial bone formation at the site of injury compared to the unseeded scaffold [38].

In a clinical case carried out by Kuroda and colleagues of tibial surgery, the efficacy of EPCs was demonstrated when autologous, granulocyte colony-stimulating factor (GCSF)-mobilized CD34(+) cells were used in successful tibial autologous bone grafting [39].
