MMP-dependent collagen-degradative activity of hMSCs. (A) RT-PCR analyses of the MMP expression profile of hMSCs cultured either on a tissue-culture plastic substratum (Plastic), atop type I collagen gels (2D; Atop collagen), or embedded within type I collagen gels (3D; Within collagen) in stem cell growth medium for 2 days. GAPDH served as the loading control. (B) Degradative activity of hMSCs (5 × 10⁴) cultured atop type I collagen films (50 μg/cm²) in the presence or absence of GM6001 (25μM), TIMP-1 (7.5 μg/mL), or TIMP-2 (2.5 μg/mL) in growth medium for 4 days. Collagenolytic activity is quantified as hydroxyproline release (mean ± SEM; n = 3). *P < .05. (C) Films of type I collagen labeled with Alexa Fluor 594 (red) were incubated with hMSCs (5 × 10⁴) stained with CELLMask plasma membrane dye (green), and zones of degradation visualized by confocal laser microscopy after a 4-day culture period. Bar, 50 μm.