Secretion of Weibel-Palade bodies observed in extra-alveolar vessels of rabbit lung

We fixed rabbit lungs by perfusion of osmium into the pulmonary artery and examined in light and transmission electron microscopy a large number of extra-alveolar vessels with a diameter of 0.1-0.25 mm, with emphasis on Weibel-Palade bodies (endothelial specific granules). Weibel-Palade bodies are organelles specific to endothelial cells. Their function is unknown, but they are useful markers for identification of endothelial cells in culture. We were able to observe release of the content of these bodies into the vascular lumen; this indicates that they are secretory.