The replacement rate of cells in our body varies greatly between different types of cell, but up until now scientists couldn't make precise estimates. Nature reports the latest results: "Samples taken from the visual cortex, the region of the brain responsible for processing sight, were as old as the subjects themselves, supporting the idea that these cells do not regenerate. ... Other brain cells are more short-lived ... In an average person of some 30 years of age, intestinal cells are about a decade old and skeletal cells a bit older than that. Cells that endure a great deal of physical stress, such as red blood cells, are known to turn over every few months. The research team believes that dating cells using carbon-14 will shed light on the role of cell death in cognitive disorders."