Mean acoustic wave speed (a surrogate measure of tissue stiffness) assessed with scanning acoustic microscopy. (A) Acoustic wave-speed distribution maps of BM-MSC- and AD-MSC-seeded type I collagen hydrogels stimulated with TGF-β or GDF6. Scale bar, 50 μm. (B) Quantification of wave speed in BM-MSC- and AD-MSC-seeded constructs after stimulation with TGF-β or GDF6. GDF6 stimulation of AD-MSCs resulted in a significantly decreased acoustic-wave speed compared with TGF-β-stimulated constructs, suggesting that GDF6-stimulated AD-MSC produced a less-stiff ECM. N = 3. *P < 0.05. (C) Wave-speed distribution in AD-MSC-seeded constructs after stimulation with either TGF-β or GDF6. The GDF6-stimulated construct shows significantly reduced mean wave speed of 1,599 ms−1 compared with the TGF-β-stimulated counterpart, 1,644 ms−1.