Perhaps the most important conceptual role for Ephrins, however, stems not from what they do, but rather from on which cells they act. While studying the functional role of Ephrins in the nervous system, Wang et al. happened to notice that the Ephrin-B2 ligand localizes to distinct subsets of blood vessels in the developing mouse embryo. Further analysis revealed that this staining pattern was a result of the differential expression of Ephrin-B2 in the arterial, as opposed to venous, endothelium of the vasculature . Subsequent studies by Shin and Garcia-Cardena et al. and Gale et al. went on to demonstrate that this Ephrin-B2 expression pattern persists into adulthood and extends not just within the endothelium, but into smooth muscle cells and pericytes as well [14, 15]. Most important, they also observed that differential arterial Ephrin-B2 expression extends into even the smallest diameter vessels, suggesting for the first time that the microvessels of capillary networks may have arterial and venous identities of their own. This differential expression was evident in vessels of multiple tissues, including pancreas, muscle, fat, kidney glomeruli, brain, liver, adrenal cortex, and adrenal medulla. Intriguingly, Ephrin-B2 also appears to differentiate arterial from venous vessels in several models of both normal and pathological angiogenesis, including marking subsets of vessels sprouting into the cornea from the limbus artery in a model of VEGF-induced angiogenesis, as well as subsets of microvessels in wounded tissue undergoing healing. Strikingly, approximately half of the vessels present in the microvasculature of different types of mouse tumor models also expressed Ephrin-B2, suggesting that the tumor vasculature may also consist of distinct arterial and venous microcirculatory components. Taken together, these studies for the first time challenged the classical view that quiescent as well as remodeling capillaries have neither arterial nor venous identities of their own, and strongly suggested that the expression of Ephrin-B2 may also be important for the formation of new vascular circuitries.
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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.