Transmission electron micrograph of a cross section through part of the liver from a trout. Cells with nuclei (colored yellow), mitochondria (colored pink) and the cytoplasm containing other organelles (colored blue) are visible. The cell nucleus houses DNA packaged into chromosomes and its function is to maintain the integrity of this genetic material and to control other functions carried out by the cell. Mitochondria are the powerhouses of the cell that generate energy. The cytoplasm is a gel like fluid that contains all of the organelles in a cell found outside the nucleus. The liver is a vital organ and helps process digested food, detoxifies the blood, and stores iron, vitamins and other nutrients. Trout are mostly freshwater fish which are a food source that can be caught wild or farmed. There are many different species including Rainbow trout and Brown trout. Horizontal width of image is 30 micrometres. Supplied by Kevin Mackenzie, courtesy of Wellcome Images.