Understanding the mechanisms of regeneration implies the ability to improve it greatly: "scientists have shown that a previously overlooked group of cells - the endothelial layer of blood vessels - is essential in helping adult stem cells multiply and revitalize damaged tissue. ... In healthy adult tissues, including bone marrow, small populations of stem cells lie dormant. The trick has been to figure out what prompts them to emerge from this hibernation-like state and start proliferating to heal damaged tissue. ... The endothelium is the innermost layer of blood vessels, made up of cells that had largely been assumed to function primarily as delivery vehicles for oxygen and nutrients. But earlier this year, [researchers] figured out that these endothelial cells also release growth factors that direct bone marrow stem cells to multiply and differentiate into different types of blood cells. Now, the researchers have shown that such ability is not limited to bone marrow but exists in the endothelium of the liver, and that it can be activated to initiate and sustain liver regeneration in adult mice. [Researchers] show that by altering the activation state of the endothelium in liver and bone marrow, they could induce adult hepatocytes and blood stem cells to divide and regenerate lost tissue. ... For the last decade, physician-scientists have been trying to transplant hepatocytes to regenerate the liver. But they grow for a few months then the majority die off. Based on our data, one could argue that just transplanting hepatocytes is not going to work. To regenerate long-lasting liver, we may need to transplant hepatocytes with the properly activated endothelium, which produces the right growth factors for the hepatocytes to attach, grow, and connect with other parts of the liver."