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Trendsetters
Leakage of plasma from the microvasculature is a hallmark of inflammation. In 1961, Majno and Palade advanced our understanding of this phenomenon when they showed, in a study using transmission electron microscopy, that histamine causes gaps to form between endothelial cells of certain blood vessels. This classic report raised numerous questions: Do gaps form at specific locations in the endothelium? How large and numerous are they? How do they form? How long do they last? How do they resolve? Are they the only mechanism by which plasma leaks from inflamed blood vessels? The present précis deals with some answers, derived through several experimental approaches.
When endothelial cell borders of leaky vessels are stained by the century-old silver nitrate method, gaps appear as stereotypic silver dots scattered along cell borders. These dots have a diameter of ~1.5 µm, and they are most numerous (~18/endothelial cell) in small postcapillary venules. The number of silver dots, like the leakage, is greatest 1 min after an inflammatory stimulus (e.g., substance P) and is back to normal by 10 min.
Three-dimensional images obtained by scanning electron microscopy show that most of the silver dots are ring-shaped deposits of silver that surround small openings ("vertical gaps") at the junction of two endothelial cells (2) (Fig. 1
). Surprisingly, the openings occupy only 5% of the total area of the silver dots and thus represent <1% (at most only 0.15%) of the endothelial surface. Most of the openings are divided into multiple pores by fingerlike endothelial cell processes (Fig. 1).
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Silver nitrate staining also marks sites where overlapping portions of endothelial cells at intercellular junctions are separated by obliquely oriented intercellular slits (2). The transendothelial pathway at these "oblique slits" is not visible by scanning electron microscopy because of the slanted orientation of the slits. Oblique slits are most numerous during gap closure, suggesting that alterations in gap morphology participate in the cessation of leakage (1).
Even though endothelial gaps are formed and closed by active cellular processes, the actual leakage of plasma is not energy dependent. Rather, the gaps are open channels permitting convective and diffusive movement of macromolecules. This fact has been shown by permeability tests on venules after fixation with glutaraldehyde-paraformaldehyde. Biotinylated Ricinus communis I lectin (molecular mass, 120 kDa) leaks through the fixed endothelium of inflamed vessels but not of normal vessels. The particulate tracer, Monastral blue B, which has a mean diameter of 0.05 µm, behaves similarly to the lectin, which is consistent with the 0.4 µm average diameter of endothelial gaps. The leakage of these substances from fixed vessels would not be explained by an alternative mechanism proposed by Dvorak et al. (J. Leukoc. Biol. 59: 100–115, 1996), involving the formation of vesiculo-vacuolar organelles and active transcytosis, but not involving endothelial gaps.
Taken together, these observations show that endothelial gaps are focal separations at cell borders. The gaps form in seconds, expose the basement membrane, permit leakage even after fixation, and can close within minutes. The mechanism(s) of gap opening and closure, as well as the role of the fingerlike cell processes, are key issues that must still be resolved. In this regard, rapid changes in intercellular junctions and in the cytoskeleton of endothelial cells are likely to be fertile areas for investigation.
References
| Occasionally, the Editor of the Trendsetters section invites contributions from the authors of published scientific articles that have been identified as being of special interest. All précis to Trendsetters are by invitation only. |
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