The shape of caveolae is omega-like after glutaraldehyde fixation and cup-like after cryofixation |
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Authors: | Wiebke Schlörmann Frank Steiniger Walter Richter Roland Kaufmann Gerd Hause Cornelius Lemke Martin Westermann |
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Institution: | 1. Electron Microscopy Center, Medical Faculty, Friedrich-Schiller-University Jena, Ziegelmühlenweg 1, 07740, Jena, Germany 2. Research Laboratory, Department of General, Visceral and Vascular Surgery, Medical Faculty, Friedrich-Schiller-University Jena, Erlanger Allee 101, 07747, Jena, Germany 3. Biocenter, Martin-Luther-Universiy Halle, Weinbergweg 22, 06120, Halle/Saale, Germany 4. Institute of Anatomy I, Medical Faculty, Friedrich-Schiller-University Jena, Teichgraben 7, 07740, Jena, Germany
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Abstract: | Caveolae were defined as flask- or omega-shaped plasma membrane invaginations, abundant in adipocytes, fibroblasts, endothelial
and smooth muscle cells. The major protein component of caveolar membranes is an integral membrane protein named caveolin.
We compared the freeze-fracture behavior of caveolae in glutaraldehyde-fixed and cryofixed mouse fibroblast cells and found
distinct differences. In glutaraldehyde-fixed cells almost all caveolae were cross-fractured through their pore and only very
few caveolar membranes were membrane-fractured. We found the reverse situation in rapid frozen cells without any chemical
fixation where most of the caveolae were membrane-fractured, showing different degrees of invagination from nearly flat to
deeply invaginated. In ultrathin sections of glutaraldehyde-fixed heart endothelial cells, caveolae exhibit the well known
omega-like shape. In high-pressure frozen, freeze-substituted and low temperature embedded heart endothelial cells, the caveolae
frequently exhibit a cup-like shape without any constriction or pore. The cup-like caveolar shape could also be shown by tilt
series analysis of freeze-fracture replicas obtained from cryofixed cells. Freeze-fracture immunolabeling of caveolin-1 revealed
a lateral belt-like caveolin alignment. These findings point out that the constricted “neck” region of caveolae in most cases
is an effect that is caused and intensified by the glutaraldehyde fixation. Our data indicate that caveolae in vivo show all
degrees of invagination from nearly flat via cup-like depressed to in a few cases omega-like. |
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