Phenotypes of stem cells from diverse origin

Stem cells have turned into promising tools for studying the mechanisms of development, regeneration, and for cell therapy of various disorders. Stem cells are found in the embryo and in most adult tissues participating in endogenous tissue regeneration. They are capable of autorenovation, often maintain their multipotency of differentiation into various tissues of their germ line and are, therefore, ideal candidates for cellular therapy taken that they can be unequivocally identified and isolated. In this review, we report stem cell marker expression used for identification of various stem cell lineages, including very small embryonic stem cells, neural, hematopoietic, mesenchymal, epithelial and limbal epithelial stem cells, endothelial progenitor cells, supra-adventitial adipose stromal cells, adipose pericytes, and cancer stem cells. These cells usually cannot be distinguished by a single stem cell marker, because their expression partially overlaps between lineages. Recent advances in flow cytometry allowing the simultaneous detection of various markers have facilitated stem cell identification for clinical diagnosis and research. So far experimental evidence suggests the existence of cells with different properties, i.e., the capability to different in various cell types. Several studies indicate that expression of classical markers for stem cell classification, such as CD34, CD45, and CD133, may differ between the virtually same stem and progenitor cells, i.e., endothelial progenitor or mesenchymal stem cells, when they were obtained from different tissues. This finding raises questions whether phenotypic differences are due to the source or if it is only caused by different isolation and experimental conditions.