Intramembrane particle distribution and junction morphology in rapidly frozen aortic endothelial cells

The morphology of unfixed plasma membranes of aortic endothelial cells from adult Sprague-Dawley rats was investigated using the Polaron E7200 quick freeze unit. Intramembrane particle distributions were determined from replicas of rapidly frozen samples and compared with counts from both aldehyde fixed, conventionaly frozen and rapidly frozen aortic samples. Aldehyde fixed cells prepared by either of the freezing methods revealed a significantly greater number of particles associated with the protoplasmic face of the membrane compared to the exoplasmic face. In contrast, those cells fixed by rapid freezing showed a nearly equal number of particles on both faces. In both fixation methods tight junctions partitioned with the exoplasmic half of the membrane, exhibiting more of a particulate morphology in rapidly frozen cells. These findings suggest that aldehydes strongly influence the morphological appearance of endothelial membranes and membrane associated structures, and that functional implications based on images of aldehyde fixed cells may be subject to errors.