Stem cells are a scientific field of interest due to their therapeutic potential. There are different groups, depending on the differentiation state. We can find lonely stem cells, but generally they distribute in niches. Stem cells don’t survive forever. They are affected for senescence. Cancer stem cells are best defined functionally, as a subpopulation of tumor cells that can enrich for tumorigenic property and can regenerate heterogeneity of the original tumor. Circulating tumor cells are cells that have detached from a primary tumor and circulate in the bloodstream. They may constitute seeds for subsequent growth of additional tumors (metastasis) in different tissues. Advances in molecular imaging have allowed a deeper understanding of the in vivo behavior of stem cells and have proven to be indispensable in preclinical and clinical studies. One of the first imaging modalities for monitoring pluripotent stem cells in vivo, magnetic resonance imaging (MRI) offers high spatial and temporal resolution to obtain detailed morphological and functional information. Advantages of radioscintigraphic techniques include their picomolar sensitivity, good tissue penetration, and translation to clinical applications. Radionuclide imaging is the sole direct labeling technique used thus far in human studies, involving both autologous bone marrow derived and peripheral stem cells. 1. Stem Cells Stem cells are a scientific field of interest mainly due to their therapeutic potential. The term of stem cells came up to us via histologists in the nineteenth century, who introduced it as a general, abstract term for cells specifically involved in repair or regeneration. With the discovery in the 1950s that bone marrow cells could reconstitute the hematopoietic systems of irradiated individuals, the modern stem cell concept began to crystallize around the experimental procedures of transplantation and reconstitution [1, 2]. The definition for tissue stem cells proposed by Potten and Loeffler was undifferentiated cells (relative to a functional tissue), capable of proliferation and production of a large number of differentiated functional progeny; they have the ability of self-maintenance of their population and for regeneration of the tissue after injury. This means that stem cells are defined by virtue of their functional attributes and not by an explicit directly observable characteristic. This functional definition is relative to the stem cell role linked to the functional tissue regeneration feature. But this definition doesn’t give us any characteristic to identify
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