Developing thrombus. Light micrograph of a section through an artery showing a newly forming thrombus (blood clot, dark pink) on the arterial wall (light pink), which has been damaged by an atheroma (fatty deposit). Damage to the wall of a blood vessel often results in the exposure of the underlying intimal collagen layer. This triggers the blood clotting process, leading to the formation of a thrombus (dark pink). Here the endothelium (inner surface) of the vessel has become ulcerated as a result.