Human stem cells used to grow a mini-liver in mice

The researchers' liver buds growing in culture prior to transplant into an animal.

Takanori Takebe

One of the things that got people excited about stem cells was the prospect that they could be used to generate entire tissues or organs, ready to replace what's been damaged by injury or disease. But there's a big gap between the embryonic stem cells we can generate and an adult organ. If you put the stem cells into an adult body, there's no way to control how they develop. But if you control their development outside of an organism, you generally wind up with a bunch of cells, mostly of a single type, sitting in a plate. That's a far cry from the complex, integrated, three-dimensional structure of an actual organ.

Now, a team of scientists in Japan has decided to split the difference. Using stem cells from various sources, they put together the three types of tissues that normally work together to give rise to the liver in embryos. And when those were implanted into a mouse, they did what they would do in the embryo: integrate together and grow into a functional liver.

In the embryo, the liver forms from a combination of two tissues. One is called endoderm, and these are the cells that line the developing gut. Resting above that is a population of loosely packed cells called mesoderm, which envelop and integrate with the endoderm to form what's called a liver bud. At that point, blood vessels become necessary for the bud to grow and specialize into the liver.