Morphological response of blood platelets to increased venular permeability in vivo

Abstract

Electron microscopy is used to investigate the response of blood platelets to isoproterenol and paracentesis-induced changes in the morphology and permeability of iridial venules in young (3–5 weeks) and older (6–8 months) rats. In isoproterenol-treated eyes of young and older rats, a notable response of the venular endothelium occurred at 1–5 min. Small patent gaps were occasionally seen between adjacent endothelial cells and numerous adlumenal protrusions and membranous vesicles of the endothelial cell were present. Individual platelets or small aggregates were closely associated with the adlumenal protrusions and membranous vesicles. Images suggestive of degranulation were seen in platelets from young animals. At 20 min, alterations of the endothelium were less prominent. In young rats, individual platelets were often surrounded by clusters of carbon particles, but were not adherent. In older rats, there was a marked adhesion of individual platelets to the endothelium in the immediate vicinity of patent gaps. Clusters of carbon particles were adhering to the platelet and dense alpha granules were characteristically present. At 2 hr, the endothelium appeared normal but in the older animal only, platelets were still adherent and associated with carbon particles. In the paracentesis experiments, patent gaps and adlumental protrusions and membranous vesicles of endothelial cells were again observed. In young rats at 20 min, small gaps were occasionally seen, and adlumenal modifications prominent. Clusters of a few platelets (less than six) were present. Within clusters platelets rarely came into close contact with one another, and possessed long pseudopods. An occasional platelet was closely associated with protrusions of the endothelium. At 1 hr the platelet response had subsided. Platelets were less numerous, and pseudopods less prominent. In older rats at 20 min, patent gaps were large and numerous. Large clusters of platelets with well-developed pseudopods were present and closely associated with carbon particles, fibrin, and a dense amorphous precipitate. Individual platelets with pseudopods were present within gaps. Degranulation was not observed. At 2–2.5 hr the endothelium had recovered. Degranulated platelets were seen adhering to the endothelium. The ultrastructural response of platelets described above, differs from that described by others in experiments designed to characterize in vivo changes in the morphology of platelets in the microcirculation. The significance of these results is discussed.