A major challenge in tissue engineering lies in generating the intricate network of blood vessels necessary for any mass of tissue. Researchers are making progress on strategies to deal with that issue, however: "Efforts to use tissue engineering to generate whole organs have largely failed primarily due to the lack of available blood vessels. Now we've essentially hijacked an existing structure to overcome this problem ... The researchers capitalized on a portion of the circulatory system shared by animals and humans called microcirculatory beds ... This is an incredible opportunity to bulk-deliver cells that don't just die. Conceivably, we could use this technique at least to supply the synthetic function of an organ by stimulating the cells to form insulin-producing pancreas cells or albumin-producing liver cells. ... The researchers concede, however, that much remains to be done before the technique could be used to generate whole organs. Indeed, [other] methods might be developed that could be more effective. But for now, they've overcome a major hurdle in tissue engineering. ... Eventually science will find a way to fabricate an organ in all its complexity. But in the short term we need to find more options for patients who are dying while waiting for transplants."