Mast cell granules. Coloured transmission electron micrograph (TEM) of a section through the granules of a human mast cell, isolated from a tumour. A tumour is an abnormal growth, which may be benign or malignant (cancerous). The granules contain heparin, histamine and serotonin, which are released in response to foreign proteins on the surfaces of invading bacteria or viruses. Heparin is an anti-coagulant & histamine increases blood vessel permeability. Each granule is surrounded by a membrane (blue), which in some places is seen as multiple layers. The interior consists of a granular matrix as well as some membranous components. Magnification: x21,000 at 6x7cm.