Effects of SF hydrogel stiffness and TGF-β1 on attachment and spreading of hMSCs. (a–h) Confocal stack renderings of hMSCs with (e–h) or without (a–d) TGF-β1 at various SF hydrogel elasticities (SF1.5-6kPa, SF2-20kPa, SF3-33kPa, SF4-64kPa). Cells were stained for cell nuclei (DAPI, blue) and F-actin (phalloidin, green), scale bar 300 µm. (i) Initial hMSC morphologies after 24 h seeding imaged and presented in grayscale with respect to SF hydrogel stiffness and TGF-β1 administration; scale bar 50 µm. Cell density (j) and attachment (k) of hMSCs for all the hydrogel matrices shown in images a–h were quantified and tabulated. * P < 0.05, *** P < 0.001, NS P > 0.05.