Stem cells in the skin: waste not, Wnt not

Human disease results from loss of organ function. Whether the tissue failure results from infarction, infection, trauma, or congenital malfunction, the ideal treatment would be regrowth of a new organ or tissue to replace that which is lost or injured. Stem cell research holds the promise of developing such treatments for many life-threatening and debilitating diseases. Stem cells come in two basic types, unrestricted and restricted. Embryonic stem cells are totipotent, able to differentiate into all cell types characteristic of the species they are taken from. Adult stem cells are thought to be more restricted in their potential, functioning as the body's natural source of cells for tissue homeostasis and repair. Although it seems unlikely that adult stem cells harvested from one tissue might be reprogrammed to take on the characteristics of a different cell type, several tantalizing results have encouraged researchers to consider this possibility. Before scientists can begin to define the limits of adult stem cell plasticity, they need to understand the signals that instruct multipotent cells to self-renew and differentiate within the lineages of their resident tissues. Skin is an excellent model system in which to explore these fundamental mechanisms, because skin keratinocytes are easily accessible and are one of the few adult stem cell types that can be maintained and propagated in vitro. These cells have already been engrafted long term to replace damaged epidermis on burn patients. Now skin biologists have begun to identify some of the key steps involved in generating a functional tissue from multipotent stem cells.

The totipotency of embryonic stem (ES) cells, coupled with their ability to respond to morphogenic signals and differentiate into any desired cell fate, makes them an attractive starting place for cell replacement therapies. If stem cells derived from adult human tissues offer similar promise, this would …