Regardless of the debate regarding moral issues of using stem cells in research, they are unequivocally useful for understanding pathological angiogenesis, particularly so in the retina. Some important stem cell concepts include a niche, as well as the ideas of self-renewal and plasticity. Self-renewal is the maintenance of a stem cell population, through production of both undifferentiated and further differentiated cells (precursors), while plasticity is the differentiation of a stem cell into various cell types. However, questions regarding plasticity exist, since cell fusion was shown to be the underlying cause for some plasticity observations. Well-studied types of stem cells include neural stem cells, mesenchymal stem cells, hematopoietic stem cells or progenitors such as endothelial precursor cells. Different cell surface markers help classify these cells types. Hematopoietic stem cells and endothelial precursor cells are involved in angiogenesis. Numerous hypoxia-regulated factors have been implicated in angiogenesis, including vascular endothelial growth factor, stromal derived factor-1, insulin-like growth factor, and monocyte chemoattractant protein-1. Progenitor cells, found amongst both early (CD34+) or late (CD14+) blood mononuclear cells, are impaired in diabetes. Studying these types of cells, along with others, can dissect the precise molecular mechanisms underlying stem/progenitor cell activity in the retina.
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