Influence of endothelial progenitor cells and platelet gel on tissue-engineered bone ectopically in goats

For the development of functional large bone tissue constructs, optimal oxygen and nutrients supply of seeded multipotent stromal cells (MSCs) is likely dependent on vascularization. The introduction of endothelial progenitor cells (EPCs) to MSC cultures might enhance vascularization and therefore increase bone formation. In this study we cocultured MSCs and EPCs and investigated performance and bone formation both in vitro and in vivo. The EPCs used were characterized by uptake of acetylated low-density lipoproteins, binding of isolectin B4 and expression of platelet endothelial cell adhesion molecule. EPC/MSC in vitro coculture showed that both cell types exerted a positive effect on proliferation of the other. For the in vivo studies, we applied platelet-leukocyte gel (PLG), containing several growth factors, as a means to further induce vascularization and thereby enhance bone formation. Cocultures and monocultures were combined with either PLG or plasma, seeded on ceramic scaffolds, and implanted intramuscularly in nine goats. After 16 weeks of implantation, it turned out that seeding MSCs and EPCs both resulted in significant more bone lining the scaffold than the unseeded controls, and MSCs and cocultures with highest MSC/EPC ratio were most competent. Cocultures did not show a higher bone content than the monoculture of MSCs. Fluorochrome incorporation results showed that the presence of seeded cells, either MSCs or EPCs, in the constructs accelerated bone formation. Finally, the addition of PLG instead of plasma did have a positive influence on the quantity of incorporated bone.