Plasmacytoma. Coloured transmission electron micrograph (TEM) of malignant cells in a plasmacytoma (plasma cell tumour). The large, irregular nucleus (granular, dark) in the cell at centre left indicates that it is undergoing mitosis (cell division). Cancer is the result of uncontrolled cell proliferation. The cell in the top right has a prominent nucleolus (brown spot in nucleus), characteristic of active cells. Plasma cells are a type of white blood cell found in bone marrow. Magnification: x2500 at 6x7cm size.