 In the endothelial lining with its basal lamina, the cells are typically flat and elongated, with their long axes oriented parallel to the direction of blood flow in the artery (Fig. 13.15). In forming the epithelial sheet, the cells are joined by tight junctions (zonulae occludentes) and gap junctions (Fig. 13.16). Endothelial cells possess rodlike inclusions called Weibel-Palade bodies that are present in the cytoplasm. These specific endothelial organelles are electron-dense structures and contain von Willebrand factor and P-selectin. Von Willebrand factor is a glycoprotein synthesized by arterial endothelial cells. When secreted into the blood, it binds coagulating factor VIII) and plays an important role in platelets’ adhesion to the site of endothelial injury. The antibody to von Willebrand factor is commonly used as an immunohistochemical marker for identification of en-dothelium-derived tumors. P-selectin is a cell adhesion molecule involved in the mechanism of neutrophil– endothelial cell recognition. It initiates neutrophil migration from the blood to the site of action in the connective tissue.
 FOLDER 13.2
Clinical Correlation: Hypertension