Mast cell granules. Coloured transmission electron micrograph (TEM) of a section through the granules of a human mast cell, isolated from connective tissue. The granules contain the chemicals heparin, histamine and serotonin. These are released in response to foreign proteins on the surfaces of invading bacteria or viruses. Heparin is an anti-coagulant and histamine increases blood vessel wall permeability. Each granule is surrounded by a membrane (green), which in some places is seen as multiple layers. The interior (purple) consists of a granular matrix as well as some membranous components. Magnification: x8750 at 6x7cm.