The technique of recellularization has been used to prepare heart valves for transplant, and here researchers are attempting the whole heart: "US researchers have revealed they made the hearts by stripping cells from the hearts of people who had died, leaving behind the organ's tough protein skeleton, known as a 'ghost heart'. The researchers seeded eight ghost hearts with living human stem cells, which successfully stuck to them and then, crucially, started turning into heart cells. ... The hearts are growing and we hope they will show signs of beating within the next week. There are many hurdles to overcome to generate a fully functional heart, but the hope is that it may one day be possible to grow entire organs for transplant. ... It follows a series of successes by [researchers] in growing beating animal hearts. The team has also taken the ghost hearts of rats and pigs and seeded them with human stem cells. Again, the cells multiplied, colonised the structure and started to beat independently. The beating strength was up to 25 per cent that of a normal heart, but the fact the hearts were beating at all was seen as a triumph."