Studies on the Intercellular Junctions of Mesothelium and Endothelium
The passage of substances across the endothelial barrier may occur—theoretically at least —by two main pathways: a) Through the cells. Particles of colloidal gold  or molecules of ferritin  injected into the plasma are taken up by the pinocytic vesicles of the endothelial cells, and thereafter appear in the extravascular spaces. This and other forms of trans-cellular passage will not be dealt within this paper. b) Between the cells. Interest in the intercellular junction as a potential pathway is stimulated by physiologic studies, which indicate that trans-endothelialpassage of ions and small molecules is a passive process. Dyes injected infra-arterially escape at discrete points along venules and capillaries , in a manner which could not be adequately explained by pinocytic transfer . It has been stated that the endothelial cells are connected by “tight junctions” or zonulae occludenles 15, 61. It has not, however, been shown that these junctions forni a continuous band around each cell, and that they do indeed represent a tight seal. The zone of membrane “fusion” could represent, rather than a seal, a very fine filter permeable to water and to very small molecules 17, 8]. By electron microscopy, the passage of tracer particles along junctions which appeared to be still “closed” has been observed : these studies were done in vitro, on rat hearts perfused with a saline solution containing particles of saccharated iron oxide (S.I.O.).
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