This study by Weibel and Palade describes two previously unknown cytoplasmic components found in the endothelial cells of small arteries and arterioles in various organs of rats and humans. The first component is a rod-shaped structure, about 0.1 μ thick and up to 3 μ long, consisting of a bundle of fine tubules embedded in a dense matrix. These tubules are ~150 Å thick and arranged parallel to the long axis of the rod. The second component is highly dilated cisternae of the endoplasmic reticulum, filled with a dense, finely granular material. These cisternae are elliptic in cross-section and can be up to 1 to 2 μ thick. The authors observed these structures in the lungs, thyroid, pancreas, intestine, heart, and skin vessels, but their functional significance remains unknown. The rod-shaped bodies were found more frequently in pulmonary arteries and alveolar capillaries, while dilated cisternae were more common in systemic arterioles. Histochemical studies suggested that the rod-shaped bodies contain a high concentration of basic groups, but no specific enzymatic activities were detected. The authors hope that further electron microscopic cytochemical studies will provide more insights into the nature and function of these structures.This study by Weibel and Palade describes two previously unknown cytoplasmic components found in the endothelial cells of small arteries and arterioles in various organs of rats and humans. The first component is a rod-shaped structure, about 0.1 μ thick and up to 3 μ long, consisting of a bundle of fine tubules embedded in a dense matrix. These tubules are ~150 Å thick and arranged parallel to the long axis of the rod. The second component is highly dilated cisternae of the endoplasmic reticulum, filled with a dense, finely granular material. These cisternae are elliptic in cross-section and can be up to 1 to 2 μ thick. The authors observed these structures in the lungs, thyroid, pancreas, intestine, heart, and skin vessels, but their functional significance remains unknown. The rod-shaped bodies were found more frequently in pulmonary arteries and alveolar capillaries, while dilated cisternae were more common in systemic arterioles. Histochemical studies suggested that the rod-shaped bodies contain a high concentration of basic groups, but no specific enzymatic activities were detected. The authors hope that further electron microscopic cytochemical studies will provide more insights into the nature and function of these structures.