The circular structure is a blood vessel and in the center there was a thrombus, that means a clot, like what happens when you have a heart attack, a thrombus then blocks a coronary artery (one that gives blood to the heart). This one is in a dog, in the skin around an area of inflammation. There are actually new blood vessels (small circular structures in the middle of the circular pink material) growing through the thrombus so that blood can start to flow again. It is called a process of recanalization. I think it is pretty cool and now you see what kind of stuff I look at for my job.