The transportation of the gas and nutrients in the blood is brought about by angiogenesis through the vascular network. It is therefore a physiologic process important to mechanisms like wound healing, menstruation, tissue growth and embryonic development (Griffioen & Molema, 2000). Endothelial cells which line the blood vessels are in direct contact with blood. Below the layer of endothelial cells, the pericytes (structural support cells) surround the blood vessels along with fibroblasts, smooth muscle cells, extracellular membrane and basement membrane (Griffioen & Molema, 2000). Endothelial cells play a role in many regulatory processes such as release of chemo-acctractants (cytokines and chemokines) that directs cells of the immune system
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In its process, there will be migration of endothelial cells, degradation of the basement membrane of the venule to move to an angiogenic stimulus, and alignment of endothelial cells to create a sprout while a curvature is formed in each endothelial cell to form a lumen in the vessel. The proliferation of the endothelial cells, increase of length of the sprout will form two hollow sprouts that join at their tips and create a loop and allow blood to flow. Pericytes will line the base of the loops and growth of new sprouts can occur. The process regarding the last steps of angiogenesis even if not clearly understood is thought to be guided by tip cells, specialized cells at the front of the sprout (Bergers & Benjamin, 2003). Gerhardt et. al. (2003) conducted a study on the angiogenesis in the retina of mice. The results found that retinal vessels grow in expanding circles and the sprouts were supported by proliferative stalk cells and guided by specialized tip cells. It revealed that the tip cells grow by extension along filopodia and that they are highly polarized with a uniquely responsive VEGF-A isoform. The filopodia are created by astrocytes and guides the tip cells by noticing the gradients of the VEGF-A. The stalk cells are also responsive to VEGF-A to its absolute concentration but not its gradient (Gerhardt, et.al,
In its process, there will be migration of endothelial cells, degradation of the basement membrane of the venule to move to an angiogenic stimulus, and alignment of endothelial cells to create a sprout while a curvature is formed in each endothelial cell to form a lumen in the vessel. The proliferation of the endothelial cells, increase of length of the sprout will form two hollow sprouts that join at their tips and create a loop and allow blood to flow. Pericytes will line the base of the loops and growth of new sprouts can occur. The process regarding the last steps of angiogenesis even if not clearly understood is thought to be guided by tip cells, specialized cells at the front of the sprout (Bergers & Benjamin, 2003). Gerhardt et. al. (2003) conducted a study on the angiogenesis in the retina of mice. The results found that retinal vessels grow in expanding circles and the sprouts were supported by proliferative stalk cells and guided by specialized tip cells. It revealed that the tip cells grow by extension along filopodia and that they are highly polarized with a uniquely responsive VEGF-A isoform. The filopodia are created by astrocytes and guides the tip cells by noticing the gradients of the VEGF-A. The stalk cells are also responsive to VEGF-A to its absolute concentration but not its gradient (Gerhardt, et.al,