Arteriole. Transmission electron micrograph (TEM) of a section through a small calibre arterial vessel called an arteriole. Several red blood cells (erythrocytes, black) can be seen inside the vessel, which is bounded by a single layer of endothelial cells. This arteriole is partly contracted hence the endothelial cell nuclei bulge into the lumen (inner space, light grey, centre). Vessels of the arterial system have an outer wall of smooth muscle cells, seen here as a single layer (outer layer). Fibroblasts, mesenchymal cells and collagen fibres make up the surrounding connective tissues. Magnification: x5600, when printed 10 centimetres wide.