Histological characterization of engineered cardiac tissue. (A, B) Nonstimulated controls. (C, D) Stimulated samples. (A, C) Mason’s trichrome staining. Collagen scaffold stains bright red (*), cells appear dark pink, nuclei appear dark blue, and cell-deposited collagen appears light blue (arrow). (B, D) Immunostaining for smooth muscle actin. Positive cells appear brown (arrow). Scale bar: 50 μm. Color images available online at www.liebertonline.com/ten.