Coloured transmission electron micrograph (TEM) of a kidney medulla showing the difference between a loop of Henle (cytoplasm, light blue; nucleus, green) and a blood capillary (yellow), Both have a very thin epithelial lining, which is hard to differentiate with a light microscope. Only the presence of red blood cells (red) can help to identify blood capillaries with a light microscope. An electron microscope shows that the epithelium of the loop of Henle is thicker the endothelium of the capillary.