 HEV cells express addressins, which are specific adhesion molecules that attach to the L-selectins on lymphocytes and anchor them to the HEV wall in preparation for crossing the endothelium.The endothelial cells of HEVs have a 'plump' appearance different from the flat morphology of endothelial cells that line other vessels, and are therefore called high endothelial cells by reference to their thickness.Another characteristic of HEVs, revealed by light-microscopic examination, is the presence of a large number of lymphocytes within their walls.