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		Background Information: Several growth factors are involved in the development of the blood system in both developing chick embryos and their yolk sacs (Gilbert, 2003). The process of blood formation, known as hematopoiesis, is divided into two categories: the embryonic stage and adult stage. This experiment investigates the embryonic stage of hematopoiesis, during which time blood vessels are created and circulation begins (Gilbert, 2003). The construction of blood cells occurs through two processes, the first of which is vasculogenesis. During vasculogenes, "blood islands" appear in the yolk sac of the embryo and form capillary networks (Gilbert, 2003). Prior to the actual formation of these "blood islands," specified mesoderm cells must first form hemangioblasts, the precursors of blood cells and blood vessels (Gilbert, 2001 and 2003). Hemangioblasts give rise to both angioblasts or hematopoietic stem cells, which form endothelial cells and blood cells, respectively (Gilbert, 2003). These cells then condense to form blood islands, the inner most cells of which are composed of hematopoietic stem cells, and the outermost cells of angioblasts (which will eventially line the inside of the blood vessels) (Gilbert, 2000 and 2003). The endothelial cells will form tubes and connect to create a network of capillaries known as the primary capillary plexus (Gilbert, 2000). The second stage of blood formation in chick embryos, known as angiogensis, is marked by the the formation of a more complex vessel system. This is accomplished by a "remodeling" of established vessels to complete the circulatory system (Gilbert, 2000 and 2003). Three paracrine growth factors (also known as cytokines) are involved in the process of vasculogeneis: basic fibroblast growth factor (bFGF), vascular endothelial growth factor (VEGF), and angiopoietin-1 (Ang1) (Gilbert, 2000 and 2003). Overall, these factors are concentrated by the extracellular matrix of the mesenchymal cells at the sites of hematopoiesis, and they contribute to blood cell and lymphocyte formation. On a smaller scale, the first growth factor, bFGF, is responisble for both the specification of mesoderm cells to form hemangioblasts, and the vascularization of the chorioallantoic membrane (CAM) tissue (Gilbert, 2000 and 2003). The CAM membrane is located beneath the shell membrane, and is formed by the fusing of the chick allantoic membrane with the mesodermal layer of the chorion (Gilbert, 2000 and 2003). It is this membrane that absorbs calcium from the eggshell, needed for the formation of the skeleton of the chick embryo (Gilbert, 2003). The second growth factor, VEGF, is secreted by the mesenchymal cells and instigates the formation of blood islands into blood vessels. Lastly, Ang1 mediates the interaction between the endothelial cells and the smooth muscle cells that cover the vessels (Gilbert, 2000) In the first of two experiments, the VEGF and bFGF growth factors were used to induce angiogenesis; both are applied to the CAM membrane of the chick embryo. In the second experiment, only the effect of bFGF on blood vessel formation (or angiogenesis) within the CAM membrane was evaluated.