Proposed Differential Signaling Pathways Activated by CG and MCG Scaffolds. CG (Col-GAG) and MCG (MC-GAG) activate different intracellular signaling pathways in MSCs during osteogenic differentiation. MCG (left) autogenously upregulates BMPR activation as detected by phosphorylation of Smad1/5/8 and gene expression of BMP-2, BMP-9, and BMP-4. This is correlated to both in vitro mineralation of MSCs and in vivo cranial defect healing. Left insert with micro-CT of explanted rabbit calvarium with regenerated bone after 12 weeks of cell-free MCG implantation. CG (right) does not activate Smad1/5/8 phosphorylation but does activate different BMP-4, BMP-7, and BMP-9 expression with ERK1/2 phosphorylation. In vitro MSC mineralization and in vivo rabbit cranial defect healing (right insert with micro-CT of explanted rabbit calvarium after 12 weeks of cell-free CG implantation) is diminished in comparison to MCG albeit detectable. Unlike MCG, CG-mediated osteogenic differentiation is improved with exogenous BMP-2 stimulation.