Currently, scientists prod stem cells to develop into specific cell types by exposing them to some of the same chemicals those cells would encounter during normal development. However, the process is often inefficient, yielding a small number of the desired cells that must then be purified from other cell types.

In the paper published last week in the journal Regenerative Medicine, West and colleagues grew embryonic stem cells under different conditions and then isolated lines of stem cells that appeared to reproduce only clones. (Embryonic stem cells, on the other hand, develop into conglomerations of nerves and other tissue when left to their own devices.) Researchers characterized the molecular markers on the different cells, identifying more than 140 unique lines. "[The] approach is very unusual," says Jeanne Loring, director of the Center for Regenerative Medicine at The Scripps Research Institute in La Jolla. "I don't think there is anyone else doing anything like this."

BioTime is already gearing up commercial manufacturing, aiming to begin shipping cells in six to 12 months. However, much work remains to be done in terms of characterizing the cells' properties. It's not yet clear how closely these cells resemble progenitor cells found in the body during normal stages of development. And follow-up studies need to be done to determine their full differentiation potential--the type of adult cell that each cell line can develop into. It's also not yet clear if these cells can be reliably reproduced, a crucial property for potential customers.