It is highly likely that, within the first decade of the new millenium, significant progress will be made in our ability to generate organs. While the average human has over 200 trillion cells, it is estimated there are only about 206 distinct cells as defined by a unique function. These cells are derived from stem cells that are pluripotent, which means that with appropriate stimulation they can develop into any kind of cell. There are two types of stem cells: embryonic and adult.22,23 Embryonic stem cells have not yet specialized into any type of cell and are obtained from two sources: (1) fetal tissues from miscarriages or abortions, and (2) in vitro embryos discarded by fertility clinics that cannot be implanted. Adult stem cells are committed to develop into a specific cell but have some limited capacity to be directed to develop into some other cell. At present, investigators have been relatively unsuccessful in obtaining stem cells from most organs in adults. Stem cells in limited numbers have been obtained from bone marrow, liver, and skeletal muscle. These stem cells, exposed to the appropriate cardiac growth factor, would be expected to develop into cardiac myocytes.24 There are already considerable preliminary data to show that fibroblasts can be transformed into skeletal muscle.25,26 The key gene that commits a cell to become a skeletal muscle cell has been identified, namely, MyoD. Transfection with MyoD has been shown to induce the phenotype of skeletal muscle in fibroblasts and several other cells.26 Myocardial infarction may be thought of as a myocyte deficiency disease in which a part of the myocardium is replaced by fibrous scar tissue. Myoblast, an adult skeletal muscle stem cell, has been transplanted with some success into the heart of a rat that had undergone a previous myocardial infarction.27 Bone marrow stem cells are being used with some degree of success in regenerating bone marrow in the treatment of leukemias. The National Institutes of Health has already developed goals to begin the pursuit of research to repair or regenerate human organs
(B-»:H:http://www.bioethics.gov). Extensive research will be required to understand the molecular factors necessary to convert stems cells into a pretargeted, specific cell. Nevertheless, this research has great potential for diseases such as myocardial infarction.
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