Confocal image of a single villus taken from a biopsy of the human duodenum (part of the small intestine). It has been stained with propidium iodide to show the healthy nuclei in orange and using the TUNEL technique which highlights the cells undergoing apoptosis (programmed cell death) in green. More dying cells can be seen further up the villus towards the luminal surface. These cells will slough off into the gut and be replaced from below.