Backaround Heart failure from Ml is a major health problem. Recent evidence suggests myocardiaf repair and recovery of LV function might be possible through tissue engineering such as autologous skeletal myoblast transplant. An alternative strategy is repair of infarcted regions with acellular biological matrix scaffolds.. -We implanted porcine urinary bladder extracellular matrix scaffolds (UBM) as full-thickness RV outflow tract replacement patches in 4 pigs and 1 dog; 1 pig received glutaraldehyde-treated pericardium as control. At 6 wk gross and microscopic structure was studied; immunohistochemistry was performed to assess cellular recruitment. m No postoperative signs of cardiac insufficiency were seen. Implant sates were pearly white at the epicardial aspect. Endocardial surfaces of UBM showed pearly white tissue interdigitated with abundant brown-colored tissue whereas GP showed mostly pearly white with sparse brown tissue. GP was a discrete foreign body in a fibrotic capsule with extensive inflammation including many giant cells. UBM was undiscernrble and the site was mixed tissue including myocardium, fibrosis, and extensive neovascularization. UBM immunohistochemistry showed myeloid cells at the endocardial aspect. Cells expressing muscle actin, with two morphologies were seen in UBM sites: stellateispindle-shaped resembling myofibrobfasts, and striated cells resembling cardiomyocytes. Neovascularization with vWfand smooth muscle actin-positive cells was abundant. Extensive replication was evident by PCNA immunostaining. The endocardium was vWf-positive endothelial cells. Conchsions UBM implants were incorporated into RV free wall by a tissue replacement process involving recruitment of circulating cells as well as appearance of muscle cells that may derive from adjacent myocardium. In contrast, GP was incorporated into a fibrotic capsule without a replacement process. Further research into potential utility of UBM for myocardial tissue repair and mechanisms of the replacement process is warranted.