Appendix L 
285 
provide a more complete understanding of early human development 
is an active area of research. 
Cardiovascular Differentiation 
Cardiovascular disease is the leading cause of death in the 
United States, taking the lives of more people each year than the 
next five leading causes of death combined [53]. Cardiovascular 
disease and its related disorders affect more than 68 million 
Americans, at a cost of more than 350 billion dollars armually. Heart 
disease alone accounts for 229 billion dollars in health care costs 
each year. Adult heart tissue cannot be expanded in culture, and 
thus, there are no human heart cell lines available for research. The 
limited amoimt of physiological research done directly on human 
heart cells has generally relied on biopsy samples, which are small, 
erratically available, and usually obtained horn diseased hearts. In 
contrast, human ES cells are already providing a reliable in vitro 
supply of human heart cells for experimental study [30-34]. 
Animal models, such as the mouse, have historically been 
used for the study of the heart. However, there are clinically 
significant physiological differences between animal and human 
cardiomyocytes that limit the usefulness of these models. For 
example, the mechanisms regulating the QT polarization interval — 
the time required for repolarization of the heart muscle between 
beats — differ significantly between species. A prolonged QT 
polarization interval in humans is related to ventricular arrhythmias 
and cardiac arrest and has been a significant side effect of a wide 
range of drugs in early human clinical trials. Drugs exhibiting this 
serious side effect must be withdrawn from clinical trials, and such 
drugs have been responsible for patients' deaths. Because the 
mechanisms that regulate repolarization of the heart muscle cells 
differ appreciably between human and mouse models, screening 
drugs on mouse hearts does not reliably detect this side effect. Yet, 
because they do not divide in culture, human heart cells have not 
been previously available for screening. 
Human ES cells differentiate spontaneously to heart muscle 
cells, and several research groups have reported the characterization 
of these cells [23, 31, 54]. Human ES cells allowed to differentiate in 
unattached clumps (termed “embryoid bodies") form synchronized 
contracting areas that express appropriate cardiac markers [23, 31, 
32]. Co-culture of human ES cells with visceral, endoderm-like cells 
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