Sasai has set his sights on more complex neural tissues. Last November, he reported the formation of a part of the pituitary gland — his “most complicated” tissue yet. In the embryo, the pituitary gland arises when two different tissues integrate to form a pouchlike structure. Sasai managed to recapitulate this in vitro partly by starting out with more than three times more embryonic stem cells than he had used to grow a mouse retina; the adjustment seems to increase the levels of signals that the cells exchange. When transplanted into mice in which the pituitary glands had been knocked out, the rudimentary organs restored the endocrine system and saved the mice. This work, too, might eventually provide a supply of pure, specialized cells, which could be used to treat endocrine disorders.