Scientists can now regenerate rat kidneys. Stripping out cells from dead kidneys to leave only a scaffold of connective tissue, they repopulate these frameworks (close up, left) with cells from humans and newborn rats, and nurture the mixtures in bioreactors designed to ape the in-body environment (right). After five days, these concoctions transform into rudimentary kidneys with all the right cells in all the right places. And when they’re transplanted into living rats, the restored organs filter toxins from blood and produce urine, albeit nowhere near as effectively as normal kidneys. The leap from proof of concept to human clinical use is huge, of course. But if the technique can be refined and scaled-up to produce adult human kidneys – perhaps by seeding scaffolds from pig kidneys, which are readily available, with human cells – it could provide life-saving replacements for people with kidney disease.