In a remarkable feat of stem-cell biology, Australian and Canadian researchers have identified a type of mammary stem cell capable of reconstituting an entire mammary gland. Their report appears in the January 5 issue of Nature. Stem cells reside within many different tissues and share two essential characteristics. First, they can give rise to progeny capable of differentiating into mature, specialized cells appropriate for their particular tissue. Often a single stem cell can ultimately produce several different types of mature cells. Second, stem cells are self-renewing, meaning that when they proliferate, they also produce more stem cells. There is considerable...
