Histological analysis on silk scaffolds with rhBMP-2/rhIGF-I reverse gradient. A–F, the first scaffold segment (lowest rhBMP-2 and highest rhIGF-I). G–L, the seventh scaffold segment (highest rhBMP-2 and lowest rhIGF-I). A,D,G,J, Hematoxylin and eosin (H&E) staining. B,E,H,K, Alcian blue staining for proteoglycan formation. C,F,I,L, Von Kossa staining for calcium deposition. Scale bars = 200 μm (A–C, G–I) or 50 μm (D–F, J–L)