Histologic analysis of co-cultured engineered heat tissue slices and engineered heart tissue at day 6. A, Macroscopic fluorescence picture of EHS (green: enhanced green fluorescent protein [EGFP]) and P-ETC (red: Vybrant DiI [Invitrogen, Karlsruhe, Germany]). B, Fluorescence micrograph of the P-ETC/EHS border zone. C, Masson's trichrome staining of the P-ETC/EHS border zone (blue: collagen, red: cytoplasm). Immunohistochemical staining for desmin (green) (D) and EGFP (green) and Cx43 (red) (E) in P-ETC/EHS border zone. Cx43 was highly expressed in myotubes in the P-ETC/EHS border zone. Scale bars = 50 μm.