The distribution of multimerin in the extracellular matrix of endothelial cells. Nonpermeabilized (a and c) and permeabilized (b and d) endothelial cells were labeled with monoclonal antibodies to multimerin (a-d), polyclonal antibodies to von Willebrand factor (b), polyclonal antibodies to fibronectin (c), and propidium iodide (to visualize cell nuclei; a), and were examined by epifluorescent microscopy. Panels a to c show paired images of the same field of primary passage endothelial cells. The cells shown in panel b contained few multimerin granules but abundant von Willebrand factor granules (N indicates a cell nucleus). Solid arrows indicate regions where the von Willebrand factor-labeled Weibel-Palade bodies in the cell periphery appeared to follow the margins of the multimerin-containing fibrils. Open arrowheads indicate multimerin in thicker, intensely labeled structures. Multimerin and fibronectin were associated with large fibrillary structures in the extracellular matrix (c), with differences in their distributions. Cell passage in vitro was associated with reduced extracellular matrix staining for multimerin (d).