Abstract: AIM To explore the practicability of constructing engineered myocardial tissues (EMTs) in vivo by cardiomyocyte-like cells derived from bone marrow mesenchymal stem cells (BMMSCs) for seeded cells and polylactic acid-co-glycolic acid (PLGA) for scaffolds. METHODS By density-gradient centrifugation, BMMSCs were isolated from the tibia and femur of Sprague-Dawley (SD) rats. BMMSCs were cultured for the third passage and cells were induced by the combination of 5-azacytidine and angiotensin Ⅱ for 24 h. Cells were cultured by replacement of the medium for 3 weeks. The differentiated cardiomyocyte-like cells were seeded on PLGA scaffolds to form grafts which were incubated in the incubator for three days, and then transplanted into the prefabricated peritoneal cavity of SD rats. After four weeks, the graft was removed and its morphology and structure were detected by HE staining, immunohistochemical staining (troponin I, cTnI) and transmission electron microscopy. RESULTS HE staining showed that the cardiomyocyte-like cells were evenly distributed in the PLGA scaffold and the nuclei were spindle shaped. The results of immunohistochemical staining demonstrated that most of the transplanted cells in PLGA-cardiomyocyte-like cells group were positive for specific protein cTnI. Transmission electron microscopy showed that in the engineered myocardial tissue constructed in vivo, the myofilaments were arranged parallel to the long axis of the cells and there were abundant mitochondria and endoplasmic reticulum in the cytoplasm. Desmosomes, gap junctions and Z line-like substances were also observed. CONCLUSION The present investigation explored a method of constructing engineered myocardial tissue in vivo. The microenvironment in vivo can help engrafted tissues or cells to survive. In vivo, engineered myocardial tissues which share similarities with the native heart tissues can be constructed in rats in vivo.