Another hypothesis regarding the potential reservoir for circulating endothelial progenitors is that they arise from the bone marrow. The potential for hematopoietic cells to contribute to endothelial formation is supported by the close relationship between endothelial cells and hematopoietic cells in development. Endothelial progenitors (angioblasts) and hematopoietic progenitors both arise from mesodermal precursors in the blood islands during early embryonic development. The angioblasts develop at the perimeter of blood islands in the yolk sac of the embryo, whereas hematopoietic precursors are formed in the center of the blood island. A common precursor for both hematopoietic precursors and angioblasts has also been described by Risau et al. in 1995 using in vivo studies in zebrafish and in vitro differentiation experiments. Embryonic angioblasts and hematopoietic stem cells also share several markers, including Flk-1, Tie-2, and CD34. These hint at the close ties between ECs and hematopoietic cells, suggesting that circulating endothelial progenitors may be generated from bone marrow cells. This line of reasoning ultimately led to the isolation and characterization of circulating EPCs.
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.