RESULTS

Normal Arterial Structure In general, preservation of arterial wall elements in both human and animal specimens was poor when the fresh tissue was fixed immediately in osmium. The internal elastic membrane was folded, and elastic fibers often appeared swollen and of decreased density. Endothelial cells were polypoid in configuration, and large vacuoles were present in smooth muscle and interstitial tissue. It was concluded that these changes resulted from contraction of the vessel and stress on the fixed tissue during polymerization of the plastic.

The best preservation of arterial fine structure was obtained in the mesenteric arteries of rats injected with osmium fixative. Endothelial cells in these arteries hada centrally placed nucleus causing the cell to bulge slightly into the lumen (Fig. I). Structures within endothelial cells were similar in rats and dogs and conformed to previous descriptions."'2'Endothelial cell cytoplasm contained vesicles measuring 40 to 6o mn in diameter. These were most numerous along the cytoplasmic membrane with which they often fused. There were also mitochondria, endoplasmic reticulum, clear vacuoles measuring up to os P, and a small Golgi zone adjacent to the nucleus. Occasional dense bodies, similar to those described by Buck,6" 8 were found. Some endothelial cells had slender cytoplasmic processes projecting into the vessel lumen (Fig. 4). The cytoplasmic membrane showed numerous areas of discontinuity interpreted as artifacts of fixation. The endothelial cell was directly apposed to the internal elastic membrane or was separated from it by unit fibers of collagen (Fig. I).