It is hardly necessary to point out the importance of studies of origin, functions, and plasticity of adult stem cells. It is a comparatively new, extremely dynamic, and rapidly developing field of research; of course, the interest it holds is primarily connected to possible therapeutic application of stem cells. Indeed, the promise of a successful replacement therapy for such diseases as Duchenne muscular dystrophy is enough of a goal to justify the effort directed toward research of muscle stem cells. The ways and means of manipulation of those cell populations need to be learned, and that means thorough knowledge of signaling proteins that direct proliferation and tissue-specific commitment of adult stem cells. The contribution of stem cells to maintenance of tissues has to be evaluated to understand the physiological relevance of those cells for normal functioning of the organism, as it was done for the hematopoietic system. Are those cells active and functioning within every tissue, and can they restore and repair it after an injury? Some of existing data point to a positive answer to this question.

Adult stem cells can also contribute to serious pathologies, at least in the case of arterial remodeling following angioplasty or in atherosclerosis (43).

Cardiovascular diseases are a serious health problem in the Western world; certainly, a way to control the pathological remodeling of blood vessels by hematopoietic stem cells that form smooth muscle cells will be a major medical achievement. This task requires understanding of mechanisms that "turn blood into muscle," of the extraordinary plasticity between those different tissue types.

Therefore, further research will be directed to analysis of the physiological role of adult stem cells in vivo and creation of experimental systems that allow the reconstruction of distinct developmental events in vitro for more detailed analysis. Understanding the molecular mechanisms that regulate the contribution of pluripotent progenitors to various tissues and organs is a key to successful manipulation, controlled changing of properties, and wide clinical application of adult stem cells.

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