Influence of scaffold properties on the inter-relationship between human bone marrow derived stromal cells and endothelial cells in pro-osteogenic conditions

One of the significant challenges in bone tissue engineering is the integration of biomaterials designed to facilitate and stimulate mineralization with a simultaneously rapid rate of angiogenesis and vascularization of the tissue construct, a challenge complicated by our lack of knowledge of the interactions among key cell types and scaffold properties. This study compared functional activity of human bone marrow-derived stromal cells (hMSC) seeded onto a porous salt-leached poly(D,L-lactic acid) (PDLLA) scaffolds, with and without the incorporation of silk fibroin fibers and then further investigated their co-culture with human umbilical vein endothelial cells (HUVECs). Cell viability, proliferation, and alkaline phosphatase activity were measured for a range of time points in culture, with osteogenic and angiogenic marker immunolocalization and gene expression at selected stages. Our findings suggest that, despite similar porosity and pore size distribution exhibited by the PDLLA and PDLLA plus silk fibroin scaffolds, there were marked differences in cell distribution and function. In the absence of fibers, a highly osteogenic response was observed in hMSCs in the scaffolds co-cultured with endothelial cells, greater than that observed with hMSCs alone or in either of the scaffolds with fibers added. However, fiber presence clearly better supported endothelial cell cultures, as determined by greater levels of endothelial marker expression at both the gene and protein level after 3 weeks of culture. The design of composite scaffolds integrating beneficial components of differing structures and materials to facilitate appropriate biological responses appears a promising yet challenging avenue of research. Statement of significance A significant challenge in bone tissue engineering is to promote a rapid vascularization of the tissue construct in parallel to the extracellular matrix mineralization. The design of composite scaffolds integrating beneficial components of differing structures and materials to facilitate appropriate biological responses appears a promising yet challenging avenue of research. Here we investigated cultures of hMSCs and HUVECs on a silk fibroin enhanced PDLLA scaffold, showing that the final output of this in vitro system is not the linear sum of the effects of the single variables. These results are of interest as they demonstrate how the addition of endothelial cells can affect hMSC phenotype and that the output can be further modulated by the introduction of silk fibroin fibers.