This is a very unusual image of a piece of synovium (the lining of the knee joint) cut through a blood vessel crossing the width of the section. It is stained with haematoxlin and eosin (the nuclei of the cells are purple) and was taken at x100 magnification.
We know that it is an arteriole (little artery) because it has a thickened wall. There are two arterioles in cross section in the top righthand corner of the image. For comparison, the double row of purple cells below the vessel is the cross section of a little vein. It has no thickened wall to maintain its shape so it has flattened so you can barely see the space within it. The little pink red blood cells (they have no nucleus) within the vessel allow us to gain some perspective – they are 8 micrometers across, so the blood vessel has a diameter of about 30 micrometers or 0.03 millimetres.
We have cut thousands of sections of this type of tissue and this was the only time we ever managed to cut a blood vessel like this.