Decellularization is a recently developed technique that allows researchers to work around the present inability to create the complex three dimensional framework that supports the cells of an organ. Here it is demonstrated in a liver: scientists "have reached an early, but important, milestone in the quest to grow replacement livers in the lab. They are the first to use human liver cells to successfully engineer miniature livers that function - at least in a laboratory setting - like human livers. The next step is to see if the livers will continue to function after transplantation in an animal model. ... To engineer the organs, the scientists used animal livers that were treated with a mild detergent to remove all cells (a process called decellularization), leaving only the collagen 'skeleton' or support structure. They then replaced the original cells with two types of human cells: immature liver cells known as progenitors, and endothelial cells that line blood vessels. The cells were introduced into the liver skeleton through a large vessel that feeds a system of smaller vessels in the liver. This network of vessels remains intact after the decellularization process. The liver was next placed in a bioreactor, special equipment that provides a constant flow of nutrients and oxygen throughout the organ. After a week in the bioreactor system, the scientists documented the progressive formation of human liver tissue, as well as liver-associated function."