A BBC News article: "It might seem unbelievable, but researchers can grow organs in the laboratory. There are patients walking around with body parts which have been designed and built by doctors out of a patient's own cells. ... There is a pressing need. A shortage of available organs means many die on waiting lists and those that get an organ must spend a lifetime on immunosuppressant drugs to avoid rejection. The idea is that using a patient's own stem cells to grow new body parts avoids the whole issue of rejection as well as waiting for a donor. ... Dr Anthony Atala [has] made breakthroughs in building bladders and urethras. He breaks tissue-building into four levels of complexity. 1) Flat structures, such as the skin, are the simplest to engineer as they are generally made up of just the one type of cell. 2) Tubes, such as blood vessels and urethras, which have two types of cells and act as a conduit. 3) Hollow non-tubular organs like the bladder and the stomach, which have more complex structures and functions. 4) Solid organs, such as the kidney, heart and liver, are the most complex to engineer. They are exponentially more complex, have many different cell types, and more challenges in the blood supply. ... We've been able to implant the first three in humans. We don't have any examples yet of solid organs in humans because its much more complex. ... One of the problems when you move to larger organs is the getting the blood supply to work, connecting arteries, capillaries and veins to keep the organ alive. It is why some researchers are investigating 'decellularisation' - taking an existing donated organ, stripping out the original cells and replacing them with new cells from the patient who will receive the organ."