QUICK SEARCH:   [advanced] Author: Keyword(s): Year:  Vol:  Page:

Cover  The endothelium of muscle capillaries is packed with plasmalemmal vesicles, later on called caveolae. These vesicles were discovered nearly 40 years ago by Palade, who hypothesized that they represent a means of transport "in quanta" across the capillary endothelium.

This electron micrograph (Bruns, R.R., and Palade, G.E. [1968]. Studies on blood capillaries. I. General organization of blood capillaries in muscle. J. Cell Biol. 37, 244-276) shows a cross-section of a blood capillary and adjacent skeletal muscle fibers in the rat diaphragm. The capillary lumen is almost completely filled with a reticulocyte (rt). The endothelium (e) is highly attenuated and contains numerous vesicles. Using such electron micrographs the authors made a three-dimensional reconstruction of a segment of the endothelial cell that showed individual vesicles and chains of vesicles opening on both the blood and tissue side of the cell. In a companion paper (Bruns, R.R., and Palade, G.E. [1968]. J. Cell Biol. 37, 299) the same authors used ferritin as a tracer and showed that the tracer was transported across the endothelium by plasmalemmal vesicles; no ferritin passed through the intercellular junctions. Based on these findings the vesicles were deemed to be the structural and functional equivalent of the large pore system. This conclusion was against the dogma of the capillary physiologists who were guided in their thinking by the pore theory of Pappenheimer, Renkin, and Borrero (1951), which held that the large pores were localized at the junctions.

The assumption that plasmalemmal vesicles are the structural/functional equivalent of the large pores is still not generally accepted by capillary physiologists despite subsequent findings by Palade, the Simionescus, and others demonstrating vesicular transport of a variety of tracers across the endothelium and the lack of permeability of endothelial tight junctions to macromolecular tracers. More recently Palade's group and others have demonstrated that the endothelial vesicles correspond to caveolae because they carry the caveolar marker caveolin in their membrane.

At present caveolae are extensively investigated in a variety of systems. They are considered to represent specialized microdomains of special lipid composition that are enriched in cholesterol and signaling molecules. The continuous microvascular endothelium is particularly rich in these structures, which appear to be specialized for transendothelial transport. This figure is reproduced from The Journal of Cell Biology, 1968, 37, p. 247, by copyright permission of The Rockefeller University Press.Marilyn G. Farquhar