It is well established that the de novo formation of blood vessels in the embryo is accomplished by the process of vasculogenesis. Endothelial progenitor cells (EPCs) or angioblasts are formed from more primitive mesodermal precursors and migrate into areas of vessel formation, where they are incorporated into the vascular wall and differentiate into mature endothelial cells (ECs). Vasculogenesis is contrasted by angiogenesis, a process classically thought to be the exclusive means of new vessel formation in the adult. Angiogenesis consists of the replication of mature endothelial cells that are resident in established blood vessels. These cells provide a supply of endothelium for the budding and branching of new vessels from the old. However, evidence has accumulated over the past decade that vasculogenesis is not limited to the prenatal period but is also an important component of new vessel growth in the adult. This chapter summarizes the current body of work on circulating endothelial progenitor cells as building blocks of adult neovasculature.
Despite the recent burst of interest in the concept of vas-culogenesis in the adult animal, evidence that an endothelial precursor is present in the adult circulation has a long history. In 1932, Hueper and Russell reported on "Capillary Tube Formation in Tissue Cultures of Leukocytes." This was followed by several reports in the 1950s of hematopoietic cells adopting characteristics of endothelial cells in vitro, and numerous publications have described the participation of blood-derived cells in the re-endothelialization of vascular grafts. Several studies suggested that the new endothelial layer was not a result of migration of cells from preexisting vessels. These early reports suggested the presence of circulating cells that could form endothelium. Several possible hypotheses could explain this finding.
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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.