Mast cell. Coloured transmission electron micrograph (TEM) of a section through a mast cell from human connective tissue. The cell has a large orange nucleus (at centre) and green cytoplasm that contains numerous red granules. Cytoplasm can be seen extending into the surrounding (blue) connective tissue. Mast cells are associated with blood vessels in the connective tissue between organs. Their function is poorly understood but they are associated with the immune response. Mast cells contain heparin, a potent blood anti- coagulant, & histamine, one of the chemical mediators of inflammation. Magnification: x14,400 at 6x7cm size.