Ultrastructural cytochemistry of trout arterial fibrils as elastic components

The trout arterial wall contains numerous extracellular fibrils that are presumed to be elastic. However, the cytochemical properties of the arterial fibrils have not been studied. Thus, we have ultrastructurally and cytochemically examined these fibrils, utilizing routine uranyl acetate and lead (UA-Pb) double staining, the tannic acid (pH 7.0)-uranyl acetate (TA-UA) method as an electron-dense staining for elastin, and Thiéry's periodic acid-thiocarbohydrazide-silver proteinate (PA-TCH-SP) method to localize vicinal glycol-containing complex carbohydrates. The arterial fibrils, about 23 nm in thickness, were interwoven at random but frequently showed the circular alignment to the long axis of the aorta. Occasionally they appeared to coalesce side by side, and the coalesced portion tended to lose its affinity for UA-Pb stains. The TA-UA method stained the fibrils moderately to intensely and stained the coalesced parts of the fibrils more intensely. All of those TA-UA positive fibrils were completely removed after elastase en bloc digestion. The PA-TCH-SP method did not stain the arterial fibrils but stained another kind of much thinner interfibrillar filamentous structure. These results suggest that the fibrils in the wall of trout ventral aorta are elastin in nature and do not contain vicinal glycols, although the fibrils usually exist in a fibrillar form, which is unlike mammalian amorphous elastin.