FIGURE 1. Can adult stem cells transdifferentiate into other cell types?

a, In previous work^{4, 5} neural stem cells, labelled with -galactosidase, were cultured with myoblast cells or embryoid bodies, producing -galactosidase-labelled muscle cells. This was interpreted as evidence that the stem cells received signals that caused them to transdifferentiate into muscle cells. But there are other possibilities. The stem cells might have been contaminated with muscle precursor cells; a few stem cells might have mutated; or the stem cells might have fused with myoblasts or embryoid-body cells. b, Ying et al.² cultured puromycin-resistant neural stem cells that expressed green fluorescent protein (GFP) with hygromycin-resistant embryonic stem (ES) cells. After selection, the surviving cells expressed GFP, were resistant to puromycin and hygromycin, and had double the normal amount of DNA (4n versus 2n). This suggests that the cells arose by fusion. c, Terada et al.³ cultured GFP-labelled, puromycin-resistant bone marrow cells with ES cells. The resulting cells expressed GFP, were resistant to puromycin and had ES-cell properties. They also had double the usual DNA content.