The endothelium, a monolayer of endothelial cells, constitutes the inner cellular lining of the blood vessels (arteries, veins and capillaries) and the lymphatic system, and therefore is in direct contact with the blood/lymph and the circulating cells. The term “endothelium” was first coined in 1865 by the Swiss anatomist, Wilhelm His [20,537]. Then, and thereafter up to the early 1970s, this monolayer was considered to be a mere diffusion barrier preventing the access of the blood cells to the vascular matrix, but it is now recognized to be a predominant player in the control of blood fluidity, platelet aggregation and vascular tone, a major actor in the regulation of immunology, inflammation and angiogenesis, and a metabolizing and an endocrine organ (Figure 1).

FIGURE 1

Multiple functions of the endothelial cells.

The endothelial cells are anchored to an 80-nm-thick basal lamina, the two constituting the intima. The basal lamina is an important component of the blood vessel as it is the scaffold of every artery, vein or capillary. The inside of this scaffold is lined with endothelial cells, while the outside is covered with smooth muscle cells or pericytes. The endothelial cells can synthesize virtually all the proteins constituting the basal lamina and produce the relevant enzymes involved in its remodeling, such as matrix metalloproteinases that degrade this extracellular matrix, an important action for the plasticity of the blood vessels and for angiogenesis [447,768].

The shape of the endothelial cells varies across the vascular tree, but they are generally thin and slightly elongated, their dimension being roughly 50–70 μm long, 10–30 μm wide and 0.1–10 μm thick. In the blood vessel wall, endothelial cells are orientated along the axis of the vessel, minimizing the shear stress forces exerted by the flowing blood. Although the endothelium is constituted by a single layer of cells, approximately 10 to 60 × 10¹² endothelial cells of the human body [690] occupy a surface (blood/endothelium interface) measuring approximately 300 to 1000 m² [727,1251]. These morphological data indicate that endothelial cells are a privileged site for exchange and transfer. However, vascular endothelial cells are not all alike and important morphologic, physiologic and phenotypic variations occur between endothelial cells in the different parts of the arterial tree as well as between arteries and veins [20,21].