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The endothelium is the simple squamous epithelium that lines the innermost layer of the cardiovascular and lymphatic vessels and is continuous with the endocardial lining of the heart. Endothelial cells are active participants in a variety of vessel-related activities, including mechanical influences on blood flow, regulation of the transport of macromolecules and blood components from the interstitium to the lumen of the vessel, secretion of chemical mediators that influence the contractile state of the overlying smooth muscle, and contribution to capillary permeability. In addition, their smooth luminal surface facilitates efficient blood flow by reducing surface friction. The vascular endothelium is divided into arterial and venous endothelia, with additional differences between larger and smaller vessels.
The fenestrated endothelium, present in organs, such as the kidney and liver, that reabsorb water and small molecules or hormones, is characterized by the presence of circular windows in the endothelial cell body. These fenestrations allow transport of water and small hydrophilic molecules. The blood-brain barrier is unique in that it is made up of a nonfenestrated endothelium. Unique tight junctions result in impermeable connections between endothelial cells, separating the cerebral capillaries from the brain.
Endothelial cells originate from the hemangioblast, a mesoderm-derived progenitor cell that gives rise to hematopoietic stem cells, and to angioblasts, the precursors of endothelial cells. These hemangioblasts then swell and form clusters, known as blood islands. These clusters exhibit the first distinction between the outer endothelial cells and the inner hematopoietic cells; the outer cells flatten and become endothelium, while the inner cells differentiate as hematopoietic cells.
Vasculogenesis and angiogenesis are two temporally separate processes which drive blood vessel development. During vasculogenesis, a network of blood vessels is created de novo by means of specification of embryonic endothelial cells, which then undergo proliferation, guided migration, coalescence, and finally, form a lumen. During angiogenesis, the primary vascular plexus is remodeled into an arborized network of large and small arteries and veins. In mammals, members of the Notch family mediate the choice of fate between arterial and venous endothelial cells; this choice is made soon after angioblast specification. The lymphatic endothelium is composed of a single layer of endothelial cells that lines the lymph vessels and is developed from venous endothelial cells. The lymphatic endothelium is more permeable than the vascular endothelium.