Background—Cardiomyocyte grafting augments myocyte numbers in the heart. We investigated (1) how developmental stage influences graft survival; (2) whether acutely necrotic or healing cardiac lesions support grafts; and (3) the differentiation and integration of cardiomyocyte grafts in injured hearts.

Methods and Results—Cardiomyocytes from fetal, neonatal, or adult inbred rats were grafted into normal myocardium, acutely cryoinjured myocardium, or granulation tissue (6 days after injury). Adult cardiomyocytes did not survive under any conditions. In contrast, fetal and neonatal cardiomyocytes formed viable grafts under all conditions. Time-course studies with neonatal cardiomyocytes showed that the grafts recapitulated many aspects of normal development. The adherens junction protein N-cadherin was distributed circumferentially at day 1 but began to organize into intercalated disk–like structures by day 6. The gap junction protein connexin43 followed a similar but delayed pattern relative to N-cadherin. From 2 to 8 weeks, there was progressive hypertrophy and the formation of mature intercalated disks. In some hearts, graft cells formed adherens and gap junctions with host cardiomyocytes, suggesting electromechanical coupling. More commonly, however, grafts were separated from the host myocardium by scar tissue. Gap and adherens junctions formed between neonatal and adult cardiomyocytes in coculture, as evidenced by dye transfer and localization of cadherin and connexin43 at intercellular junctions.

Conclusions—Grafted fetal and neonatal cardiomyocytes form new, mature myocardium with the capacity to couple with injured host myocardium. Optimal repair, however, may require reducing the isolation of the graft by the intervening scar tissue.