Ultrastructural observations on Weibel-Palade bodies suggesting exocytosis

Weibel-Palade bodies were studied in endothelium in human tumours by transmission electron microscopy of ultrathin sections. In addition to individual Weibel-Palade bodies lying in the cytoplasm, one structure was observed which was interpreted as a Weibel-Palade body and in which the limiting membrane was in continuity with the plasmalemma, suggesting exocytosis. The interior of non-exocytosing Weibel-Palade bodies was generally rather dense and the microtubular inclusions well delineated. Some, however, were of reduced density and had fewer and less well delineated microtubular elements. This appearance corresponded with that of the putatively exocytosing Weibel-Palade body. It was suggested that the microtubular elements re-organise and possibly disorganise with the approach of endocytosis.