The effect of the microenvironment created by a titanium mesh cage on subcutaneous experimental bone formation and...

We attempted to form ectopic bone under the skin of rats without adding any extrinsic bone-inducing growth factors or cytokines using bone marrow stromal cells (BMSCs), a collagen scaffold and a titanium mesh cage. We set up a space made up of a cage inserted into the subcutaneous region of rats' backs, where we could eliminate the possible influence of residual bone tissue on bone induction. We filled this space with a collagen matrix containing BMSCs. At week 8 and month 6 after implantation, the specimens were removed and observed histologically, histochemically and enzyme histochemically. As a result, bone tissue was identified in each case within the titanium cages, even though we had not used bone-inducing chemical substances. Bone generation was not found in test cases without a cage. Enhanced green fluorescence protein (EGFP) labeling of the implanted BMSCs clearly showed that these cells differentiated into osteoblasts and subsequently into osteocytes in the formed bone tissue. Host cells without EGFP labeling were also confirmed to be involved in bone formation. Six months after transplantation, the implanted cells were still present in the generated bone, and no significant resorption of the generated bone was observed. These results indicate that the physically stable spatial microenvironment created by the cage in vivo plays an important role in bone formation and inhibition of its resorption, which we refer to as the 'cage effect'.