[Tumor stem cells and "niche"--a limiting factors of the carcinogenesis].

Presently, the view of tumor development is increasingly linked up with disorders of stem cells, no matter whether they are normal tissue stem cells or early progenitor cells. There is much evidence now that microenvironment regulates tissue specificity, reparation process as well as process of carcinogenesis. The aim of this paper is to enlarge the basic knowledge of carcinogenesis and show the role of stem cells and tumor stem cells as potential primary source of tumor development. We support the view that tumor may be considered an abnormal "organ" where the growth of tumor cells is controlled by a rare subpopulation of tumor stem cells giving rise to both greater number of tumor cells and non-tumorigenic tumor cells. This all is complementary and in connection with microenvironment we can speak about limiting factors of carcinogenesis. This insight into the tumor biology shows the shortcomings of recent tumor therapy. Therefore, most likely for effectual tumor therapy, attention should be given to both tumor stem cells and microenvironment. Consequently, this complementary relationship should be considered a limiting factor for understanding of carcinogenesis and also for further therapeutic strategy--homing therapy (tailored therapy) with the use of stem cells.