Electron Microscope Study of Elastase-Digested Rat Aorta

Abstract

Ligated lengths of normal rat aortae were injected with elastase and suspended in borate buffer, pH 8.6, for 15, 30 and 60 minutes at room temperature. Controls were treated with the buffer alone. All specimens were then fixed and examined in the light and electron microscopes. The buffer-treated tissue appeared normal histologically, but the electron microscope revealed some change in the smooth muscle cell cytoplasm, altered desmosomes, and an apparent loss of "ground substance" around the cells accompanied by exposure of the branches of elastin as they left the laminae. These branches were composed of filaments 160-190 A in diameter that exhibited an increased electron density due to a greater affinity for the lead used to stain the ultrathin sections. The enzyme digested the elastin and muscle cells in concentric layers from within outwards, nearly all the aortic elastin being solubilized after one hour. The desmosomes were again altered and partially separated from the laminae, and the possible implication of this finding on the formation of aortic aneurysms in vivo is discussed. The digested edges of the laminae were denser than their unstained counterparts, and the enzyme attacked the filamentous component of elastin to give a leadstained, reticular network. This confirms, in sectioned material, the dual nature of elastin suggested previously by chemical and enzymatic studies, and by the morphology of enzyme-treated, fragmented tissue.