Adult cells can be reprogrammed in vivo according to a recent report in Nature (502, 340–345).

Although reprogramming of somatic cells into induced pluripotent stem (iPS) cells can be achieved in vitro by ectopic expression of four factors (Oct4, Sox2, Klf4 and c-Myc), this had not been demonstrated in vivo.

Manuel Serrano and his colleagues now describe a mouse model in which administration of doxycycline induces the expression of these four reprogramming factors. Mice fed doxycycline developed teratomas that contain the three embryonic germ layers—mesoderm, endoderm and ectoderm—suggesting that the reprogrammed cells are fully pluripotent. Pluripotent cells were found in the stomach, intestine, pancreas, kidney and circulating in the blood of the reprogrammed mice. Reprogramming could also be induced in hematopoietic cells, as transplantation of bone marrow from the reprogrammed mice into wild-type hosts resulted in teratoma formation. The in vivo–generated iPS cells were more similar to embryonic stem cells than in vitro–generated iPS cells and could produce embryo-like structures after their injection into wild-type mice. Looking forward, these mice will provide a valuable tool to investigate reprogramming in situ and may point to novel strategies for promoting regeneration.