Rough endoplasmic reticulum and nucleus. Coloured transmission electron micrograph of the nucleus (green) and rough endoplasmic reticulum (ER), coloured red, in a mammalian cell. ER is a complex membrane system, originating from the nuclear membrane, that divides the cell cytoplasm into a network of cavities called cisternae. The surface of rough ER is covered with bodies called ribosomes, (black dots on the membrane). Ribosomes on ER (as opposed to free-floating ribosomes) produce proteins that are secreted outside the cell. Mitochondria are turquoise. Magnification: x3850 at 6x6cm size. Magnification: x4,500 at 4x5 inch size. Magnification: x3,900 at 6x7cm size.