WITHIN THE BODY, normal stem cells appear to be able to divide for the lifetime of the organism because they exist in a microenvironment called a niche. These niches provide tissue support (adult stem cells are often on the basement membrane, surrounded by specific cell types called stromal cells) and signaling (between niche and stem cells, hormonal, neural, and metabolic pathways) that control the action of stem cells in a dynamic that sustains the tissue. With few exceptions, stem cells always remain in the niche and may be attached by adhesion molecules.

Researchers at the Howard Hughes Medical Institute identified the types of cells that make up the niche in Drosopbila by altering/marking individual stem cells. They learned that stem cells could migrate to a new ...