Tumour blood vessels. Coloured scanning electron micrograph (SEM) of a resin cast of blood vessels from an intestinal tumour. This branching network of vessels surrounded the tumour, supplying it with blood. Tumours are the uncontrolled growth of a tissue, and they trigger a growth of new blood vessels to supply the blood for such growth. The blood vessels form a chaotic structure here, as opposed to the normally ordered structure found in the intestines. Tumours may be malignant, invading and destroying other tissue as a cancer. This cast was made by injecting resin into the blood vessels and then chemically digesting the tissue.