ABSTRACT
In the living tissues of pluricellular organisms, cells are attached to each other either directly by molecules embedded in the plasma membrane, or through the extracellular matrix, a mixture of polymers secreted by cells, onto which they adhere. Blood vessels are a good example of such complex organized structures. They are made
of three layers. First, in contact with the blood flow, a monolayer of endothelial cells provides an impermeable barrier to most molecules and cells, but allows the transport of specific molecules (e.g., glucose) by transcytosis and the transmigration of specific cells (e.g., monocytes) between the lumen and the vessel wall. In this very thin layer (a few µm), cells are attached to each other by adherens junctions. Second, layers of smooth muscle cells, organized around the endothelial cell monolayer, control the vessel diameter in response to mechanical stimuli (blood pressure, blood flow shear stress). These are relayed by chemical mediators secreted by the endothelial layer. For example vasodilation and vasoconstriction are regulated by nitric oxide and angiotensin, respectively. Depending on the vessel size, the smooth muscle cell layer thickness varies from 50 µm to several mm. Third, an external layer made of elastic fibers and fibroblast cells ensures mechanical stiffness. In large vessels, this layer may be a few millimeters thick and also contains nutritive vessels and nerves that stimulate smooth muscle cells. A specific 80 nm thick extracellular matrix layer, called the basal membrane, separates the endothelial cells from smooth muscle cells [1]. Another specific extracellular matrix layer separates the smooth muscle cell layer from the external layer. This example shows that in living tissues, several differentiated cell types co-exist in oriented structures organized at different scales, from about 10 nm to several mm.
