Morphology of rapidly frozen aortic endothelial cells. Glutaraldehyde fixation increases the number of caveolae.

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Abstract
Using a Polaron E7200 quick freeze unit, we investigated the distribution of vesicles (caveolae) bound to the plasma membrane of aortic endothelial cells from adult Sprague-Dawley rats. Counts of caveolae from replicas of rapidly frozen, unfixed samples were made and compared with counts from both aldehyde-fixed, conventionally frozen, and rapidly frozen aortic samples. Aldehyde-fixed samples prepared for freeze fracture by either of the freezing methods revealed a significantly greater number of caveolae than did unfixed, rapidly frozen samples. These findings suggest that the relative number of caveolae is artifactually increased by exposure to aldehyde fixatives. We conclude that previous estimates of the rates of pinocytotic activity in vascular endothelial cells may contain substantial errors based on overestimates of caveolae and underestimates of subplasmalemmal "free" vesicles.