Cell nucleus. Transmission electron micrograph of the nucleus (circular) of a mouse liver cell. Surrounding the nucleus is delicate nuclear membrane, which contains gaps called nuclear pores that allow large molecules to pass out into the cell cytoplasm. The dark area in the lower part of the nucleus is the nucleolus. This contains unravelled chromosomes involved in making protein- manufacturing bodies called ribosomes. Outside the nucleus are numerous energy-producing mitochondria (black) and membranes of the cell's endoplasmic reticulum (centre right, for example), which is studded with tiny black ribosomes. Magnification x14,250 at 10x8 inch size.