Human embryonic stem cells are human cells, that may be stably multiplied and cultured in vitro, that are at least pluripotent and may be totipotent. By that it is meant that the cells can differentiate into many different mature differentiated cell types of the human body and may, in fact, be able to differentiate into all of the cell types of an adult human body. Human embryonic stem cells are created from embryonic tissues and serially cultivated thereafter in an in vitro culture.
In cultivation, human embryonic stem cells are normally maintained in an undifferentiated state by culturing in conjunction with certain factors. Notably, the cultivation of human embryonic stem cells upon fibroblasts feeder layers, or in the presence of factors derived from fibroblasts, maintain the stem cells in an undifferentiated state. With the fibroblasts or the factors from the fibroblasts removed, human embryonic stem cells can and will begin to spontaneously differentiate into a variety of tissue types. Among the intermediate structures formed by stem cells in the process of spontaneous differentiation into a variety of tissue types is a structure known as an embryoid body. Embryoid bodies begin as aggregates formed in the culture of embryonic stem cells. While culture conditions and cell line identity influence the rate formation of embryoid bodies, under many conditions, embryoid bodies will both spontaneously arise and spontaneously begin to differentiate into a variety of different tissue types.
Among the tissue types present in embryoid bodies are known to be cardiomyocytes. These early cardiomyocytes are the precursors of human adult cardiac cells. Adult cardiomyocytes permanently withdraw from the cell cycle and cannot regenerate. The fact that cardiomyocytes were among the cells present in the embryoid bodies formed by stem cells was evident by the fact that parts of the embryoid bodies will sometimes exhibit regular heartbeat-like contractions. Thus it has been previously demonstrated that human embryonic stem cells will differentiate into cells which have some of the functional properties of cardiomyocytes. Exactly what form those cardiomyocytes take, and how mature they are in their differentiation, was previously unknown. Also unknown was what electromechanical mechanisms are active in the cardiomyocytes present in embryoid bodies and what sorts of analysis of the behavior of those cardiomyocytes cells derived from stem cells can be performed.