25 years ago, a group of scientists embarked on an audacious quest: the creation of whole new organs. Brothers Joseph and Charles Vacanti at Harvard Medical School and Robert Langer of the Massachusetts Institute of Technology first promoted the idea of "tissue engineering" or "regenerative medicine". The scientists knew that every organ has a “scaffolding” — a structure that gives it shape — and many different types of cells with different functions. There are millions of cells, all arranged in an exact order. Langer was a master at making physical structures that live within the body. He devised ways to create an organ's scaffolding using a variety of synthetic materials like biodegradable polymers and natural molecules like collagen that are part of everyone's body. Using Langer's techniques, Charles Vacanti grew cartilage in the shape of the ear of a 3-year-old child. Tissue engineers also devised an ingenious alternative approach called "decellularization": the cells of a spare organ (from an animal or human cadaver) are digested away, leaving just the natural, noncellular scaffolding. Once a scaffolding exists, the next step in building a new organ is to drape the scaffolding with cells. Where do the new cells come from? Most often they are adult stem cells from the type of organ the doctors want to build. Sometimes adult stem cells from other organs can be chemically coaxed to turn into the cells of the new organ. For example, in pregnant women the amniotic fluid and placenta contain stem cells that can form many tissues. Surprisingly, so does fat. The unwanted fat removed by liposuction, for example, is rich in stem cells. Ideally adult stem cells are extracted from the person who will be receiving the new organ, so that it won't be rejected. Because adult stem cells have so far worked well, scientists haven't made much use of the more controversial embryonic stem cells or the recently created induced pluripotent stem (iPS) cells that seem to behave much like their embryonic cousins. After the scaffolding came the hard part, the part that caused most scientists outside the field to predict that growing new organs would fail. Even if you could build a scaffolding and procure the cells to drape onto that scaffolding, then what? Surely no scientist could assemble millions of cells, one by one and each in the right place, as if the organ were a giant jigsaw puzzle. It was, on its face, an insoluble problem — unless nature, somehow, helped out.