Amultipotent skin-derived stem cell, and a purified neural stem cell from the brain have come on the scene at a delicate time in the debate over embryonic stem cell research. Each success in isolating adult stem cells, such as these cells, gives ammunition to opponents of embryonic stem cell research, who point to adult-derived stem cells as an alternative source of transplant-ready material.
The skin cells come from the work of Freda Miller and colleagues at McGill University, Montreal, Canada. Miller was prompted to look for neural precursors in skin based on the fact that certain mechanosensing neurons appear to be born in the skin. She relied on a protocol used to isolate neural stem cells from the brain, which involves purifying floating clusters (or spheres) of cells away from adherent cells. The purified spheres yielded various cells with characteristics of neurons, glia, smooth muscle cells, and adipocytes, depending on the nature of the added serum. The same range of cell types resulted when single cells were first isolated and grown up before the addition of serum.
The brain neural stem cells come from Perry Bartlett's group at the Walter and Eliza Hall Institute (Parkville, Australia), after a search in two areas of the brain previously implicated in neural stem cell production. Sorting on the basis of the cells' ability to form neurospheres, they found a cell size and marker phenotype that includes 0.27% of the original cells, but within which 80% of the cells can generate neurospheres. All clones can form neurons, oligodendrocytes, astrocytes, and, when cocultured with a muscle cell line, myocytes and myotubes. Thus, the two studies are in agreement that a single adult stem cell can generate both muscle and neural cells.
For therapeutic purposes, Bartlett's cells can be purified but are still inaccessible. Miller, however, managed to get significant numbers of her skin-derived stem cells from human scalp. “The fact that we can get anything at all from the tiny scalp samples is very encouraging,” she says.
As adult stem cells get purer and reveal greater plasticity, the unique claims of embryonic stem cells to pluripotency (the ability to form all cell types) are being whittled away. But Miller is not ready to declare victory yet. “I think adult stem cells will certainly be a viable option for certain kinds of diseases,” she says. “The question is how broad that will be. We are working like crazy to ask how flexible [these stem cells] can be.” ▪