Effect of serum on human bone marrow stromal cells: Ex vivo expansion and in vivo bone formation

Background. Bone marrow stromal cell (BMSC) transplantation may offer an efficacious method for the repair of bone defects. This approach has been developed using BMSCs expanded ex vivo in medium with fetal bovine serum (FBS), For clinical applications, however, contact of BMSCs with FBS should be minimized. We studied the effect of FBS substitutes on both human BMSC proliferation in vitro and subsequent bone formation in vivo. Methods. BMSC proliferation was measured by colony forming efficiency (CFE) and by cell numbers at consecutive passages. Bone formation was studied in 6- to 8-week-old transplants of human BMSCs in immunocompromised mice. Results. Medium with FBS was more effective in stimulating BMSC proliferation than medium with either human serum (HS) or rabbit serum (RS), Compared to bone formed by BMSCs cultured continuously with FBS, bone formed by cells cultured with HS, or with FBS switched to HS, was considerably less extensive, while bone formed by cells cultured with FBS witched to serum-free medium (SFM) was considerably more extensive. The increase in bone formation was due to neither the SFM components nor to the proliferation status of BMSCs prior to transplantation. Conclusions. Our data demonstrate that for ex vivo expansion of human BMSCs, medium with FBS remains most effective. However, incubation of human BMSCs in SFM prior to in vivo transplantation significantly stimulates subsequent bone formation. This finding increases the practicality of using culture-expanded BMSCs for autologous human transplantation and suggests the presence of osteogenic inhibitors in serum.