Human embryonic stem cells can differentiate into myocytes with structuraland functional properties of cardiomyocytes

The study of human cardiac tissue development is hampered by the lack of a suitable in vitro model. We describe the phenotypic properties of cardiomyo cytes derived from human embryonic stem (ES) cells. Human ES cells were cul tivated in suspension and plated to form aggregates termed embryoid bodies (EBs). Spontaneously contracting areas appeared in 8.1% of the EBs. Cells f rom the spontaneously contracting areas within EBs were stained positively with anti-cardiac myosin heavy chain, anti-alpha -actinin, anti-desmin, ant i-cardiac troponin I (anti-cTnI), and anti-ANP antibodies. Electron microsc opy revealed varying degrees of myofibrillar organization, consistent with early-stage cardiomyocytes. RT-PCR studies demonstrated the expression of s everal cardiac-specific genes and transcription factors. Extracellular elec trograms were characterized by a sharp component lasting 30 +/- 25 millisec onds, followed by a slow component of 347 +/- 120 milliseconds. Intracellul ar Ca2+ transients displayed a sharp rise lasting 130 +/- 27 milliseconds a nd a relaxation component lasting 200-300 milliseconds. Positive and negati ve chronotropic effects were induced by application of isoproterenol and ca rbamylcholine, respectively. In conclusion, the human ES cell-derived cardi omyocytes displayed structural and functional properties of early-stage car diomyocytes. Establishment of this unique differentiation system may have s ignificant impact on the study of early human cardiac differentiation, func tional genomics, pharmacological testing, cell therapy, and tissue engineer ing.