The three-dimensional organization of plasmalemmal vesicular profiles in the endothelium of rat heart capillaries.

The organization of plasmalemmal vesicular profiles in the endothelium of rat heart capillaries has been reinvestigated. Judged from random thin sections approximately 50% of the vesicles appeared free in the cytoplasm, the rest opening to the surfaces of the endothelial cells--a distribution which corroborates previous studies. However, three-dimensional reconstructions based on ultrathin serial sections (thickness congruent to 12 nm) gave a very different picture. All plasmalemmal vesicular profiles (921 from 5 capillaries) were parts of the surface membrane either as caveolae or as more complex racemose invaginations. This organization has previously been observed in frog mesenteric capillaries ((M. Bundgaard, J. Frøkjaer-Jensen, and C. Crone, 1979, Proc. Nat. Acad. Sci. USA 76, 6439-6442) and (J. Frøkjaer-Jensen, 1980, J. Ultrastruct. Res. 73, 9-20)). It is therefore proposed that absence or extreme rarity of free plasmalemmal vesicles is a general feature of capillary endothelia. Consequently, we suggest that the term "endothelial, plasmalemmal vesicles" be replaced by "endothelial plasmalemmal invaginations." The results imply that trans-endothelial vesicular transport is unlikely to occur and that this membrane system performs other--as yet unknown--functions.