Immunoelectron microscopy studies of human umbilical vein endothelial cells. (a) Cells labeled with multimerin antibodies. Gold particles were located in round dense-core granules (arrows) and less often in elongated structures (arrowheads) that closely resembled Weibel-Palade bodies. (b and inset) Cells double immunolabeled for multimerin (10 nm gold) and P-selectin (15 nm gold). Multimerin immunolabel was mainly observed in dense-core granules (arrows) that were negative for P-selectin. P-selectin– positive Weibel-Palade bodies (WP; arrowheads) displayed either low levels (b) or no detectable (inset) multimerin immunolabel. N indicates the cell nucleus.