The concept of regenerating diseased myocardium by implantation of tissue-engineered heart muscle is intriguing, but convincing evidence is lacking that heart tissues can be generated at a size and with contractile properties that would lend considerable support to failing hearts. Here we created large (thickness/diameter, 1-4 mm/15 mm), force-generating engineered heart tissue from neonatal rat heart cells. Engineered heart tissue formed thick cardiac muscle layers when implanted on myocardial infarcts in immune-suppressed rats. When evaluated 28 d later, engineered heart tissue showed undelayed electrical coupling to the native myocardium without evidence of arrhythmia induction. Moreover, engineered heart tissue prevented further dilation, induced systolic wall thickening of infarcted myocardial segments and improved fractional area shortening of infarcted hearts compared to controls (sham operation and noncontractile constructs). Thus, our study provides evidence that large contractile cardiac tissue grafts can be constructed in vitro, can survive after implantation and can support contractile function of infarcted hearts.

Background— Cardiac tissue engineering aims at providing heart muscle for cardiac regeneration. Here, we hypothesized that engineered heart tissue (EHT) can be improved by using mixed heart cell populations, culture in defined serum-free and Matrigel-free conditions, and fusion of single-unit EHTs to multi-unit heart muscle surrogates. Methods and Results— EHTs were constructed from native and cardiac myocyte enriched heart cell populations. The former demonstrated a superior contractile performance and developed vascular structures. Peptide growth factor-supplemented culture medium was developed to maintain contractile EHTs in a serum-free environment. Addition of triiodothyronine and insulin facilitated withdrawal of Matrigel from the EHT reconstitution mixture. Single-unit EHTs could be fused to form large multi-unit EHTs with variable geometries. Conclusions— Simulating a native heart cell environment in EHTs leads to improved function and formation of primitive capillaries. The latter may constitute a preformed vascular bed in vitro and facilitate engraftment in vivo. Serum- and Matrigel-free culture conditions are expected to reduce immunogenicity of EHT. Fusion of single-unit EHT allows production of large heart muscle constructs that may eventually serve as optimized tissue grafts in cardiac regeneration in vivo.

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