Transmission electron micrograph (TEM) of a human mast cell in connective tissue. The large oval structure in center is the nucleus, containing the cell's genetic material. The cytoplasm contains numerous granules (dark stain) which are released in response to inflammation or allergy. The granules contain heparin, histamine & serotonin. Heparin is an anti-coagulant; histamine increases the permeability of blood vessel walls. Release of granule contents probably occurs where there is a tendency for blood clot formation, or as part of the body's general defense mechanism at sites of infection.