The vasculature forms via two processes: vasculogenesis and angiogenesis. In vasculogenesis, endothelial cell (EC) precursors called angioblasts associate to form early vessel tubes. Tissues that are vascularized by this process are generally of endodermal origin and include lung and pancreas as well as the heart tube and dorsal aorta. In the process of angiogenesis, small blood vessels form by budding and sprouting from larger, extant vessels. Tissues of ectodermal and mesodermal derivation, such as the kidney, brain, and retina, are thought to be vascularized primarily via angio-genesis. In addition, angiogenesis appears to be the predominant means of neovascularization during events such as wound healing and during pathologies such as proliferative diabetic retinopathy. Whether vessels form by angiogenesis or vasculogenesis, the primitive vessels are subsequently remodeled. Remodeling is a poorly understood event that involves growth of new vessels and the regression of others, as well as changes in lumen diameter and vessel wall thickness, to suit the local tissue needs.
Although there are fundamental differences between angiogenesis and vasculogenesis, these two processes share several common regulatory features.
Until recently, information regarding the regulation of angiogenesis and vasculogenesis came from classic embry-ological studies. During the past decade, cell and molecular analyses have divulged several vessel-specific receptors and their corresponding high-affinity ligands. These include the isoforms of VEGF and its receptors, flkl, fltl, and neu-ropilin-1; platelet-derived growth factors A and B (PDGF) and their receptors a and b; angiopoietins (ang) 1 and 2 and their receptor tie-2; and transforming growth factor b (TGF-b) and the TGF-b receptors 1 and 2 [1-3]. This database, when combined with the ability to genetically manipulate vascular development and differentiation using mouse genetic approaches, has begun to provide important insights into the molecular mechanisms that regulate vessel growth and remodeling. Although our knowledge of the molecules involved in the assembly of mature, quiescent blood vessels is far from complete, observations to date do permit some speculation regarding how vascular cells take advantage of these developmental and differentiative signaling pathways to assemble a physiologically functional vasculature [1, 3].
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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.