The endoendothelial lining as studied by a fluorescent labeling technique in situ.

The microcirculatory blood vessels of the exposed rat mesentery were observed by intravital microscopy using incident fluorescence excitation and transmitting bright field illumination. Fibrinogen conjugated with the fluorescent dye Fluorescein-Isothiocyanate and injected intravenously was found to accumulate at the vessel wall mostly at the inner lining of venules. The accumulation did not occur in an uniform manner, but appeared as circumscribed streaks, nets and bands in some areas whereas other parts of the microcirculation remained free of any fibrinogen wall adherence. Heparin pretreatment did not influence the phenomena. Blocking the fibrinolytic activity of the blood increased the fibrinogen deposition. Fibronectin labeled with a second fluorescent dye, Rhodamin B-Isothiocyanate, could be differentiated from fibrinogen in the same animal. Both proteins accumulated at the same places of the vessels inner lining. Since albumin and gamma globulin did not show any affinities to the vessel wall, we postulate receptors for fibrinogen and fibronectin at the endoendothelial surface of the microcirculatory endothelial cells.