Stem cells branch out

Differentiation of embryonic stem (ES) cells, which are originally totipotent, puts increasing restrictions on the final fates that a cell can achieve. This simple idea was upset last year when neural stem cells were shown to produce blood cells in irradiated adult mice. In the 2 June issue of Science, Clarke et al. show that neural stem cells injected into embryos can generate a wide variety of tissues including cells in the central nervous system, heart, liver, and intestine (Science 2000, 288:1660-1663). This raises the possibility of using similar stem cells for human therapy, in place of the ethically questionable use of human ES cells.