Immunoelectron micrographs of human umbilical vein endothelial cells double-immunolabeled with antibodies to multimerin (10 nm gold) and von Willebrand factor (15 nm gold). (a) Most of the multimerin-positive granules were round in shape (arrows) and distinct from the von Willebrand factor-containing elongated Weibel-Palade bodies (WP). (b) Two dense-core granules of similar size with distinct appearances and contents. One is elongated and contains von Willebrand factor (arrowheads) whereas the other one is more spherical and contains multimerin (arrows). (c) Weibel-Palade (WP) bodies containing von Willebrand factor were generally devoid of multimerin (arrowheads). Occasionally, both proteins were found in their matrix, but the labeling for multimerin (arrows) was minimal compared to von Willebrand factor.