Fibrillar collagen content visualized by using polarized light microscopy of picrosirius red-stained sections. (A) Picrosirius red staining of MSC-seeded type I collagen hydrogels stimulated with TGF-β or GDF6, demonstrating enhanced fibrillar collagen deposition (red and green stains) in AD-MSC-seeded constructs stimulated with TGF-β compared with GDF6-stimulated constructs. Scale bar, 500 μm. (B) Quantification of percentage fibrillar collagen content in BM-MSC- and AD-MSC-seeded constructs after stimulation with TGF-β or GDF6. AD-MSC-seeded constructs stimulated with TGF-β demonstrated a significantly higher percentage of fibrillar collagen content compared with GDF-6-stimulated constructs; N = 3.