Heterogeneity of endothelial cells. Specific markers.

During embryonic development, endothelial cells differentiate from a common precursor called angioblast and acquire organ-specific properties. One of the important determinants of endothelial cell differentiation is the local environment, and especially the interaction with surrounding cells. This interaction may occur through the release of soluble cytokines, cell-to-cell adhesion and communication, and the synthesis of matrix proteins on which the endothelium adheres and grows. The acquisition and maintenance of specialized properties by endothelial cells is important in the functional homeostasis of the different organs. For instance, in the brain, alteration of the blood-brain barrier properties may have important consequences on brain functional integrity. One of the major limitations to the study of endothelial cell heterogeneity is the fact that these cells are still difficult to isolate and culture from the microcirculation of different organs, and once in culture, they tend to lose their specialized properties. This finding suggests that we have to develop new culture systems, which possibly include coculture with other cell types. An important issue is to develop tools that can help in recognizing endothelial cells and their differentiated phenotype both in vivo and in tissue culture. In this review we give a short overview of the differentiated properties of the endothelium, considering a few examples of highly specialized endothelial cells, such as the brain or bone marrow microcirculation or high endothelial venules. We made a particular effort to list the most common markers of endothelial cell phenotypes. These molecules and related antibodies may be valuable tools for endothelial cell isolation and characterization.