Lately, we’ve made much of the stem cell niche, the combination of tissue microenvironment and signaling factors in which stem cells thrive (or not). In both human and fly, the niche is critical to proper stem cell functioning. As we age, stem cells begin to fail; an accumulating body of evidence suggests that alterations in the niche are partially responsible.

But what is the stem cell niche, exactly? (Since there’s one for each type of stem cells, I should have asked “what are stem cell niches?”) If you’re wondering the same thing, you’re in luck: Here’s a big review of the niche in multiple organisms, by Morrison and Spradling. The piece includes not only a thorough discussion of age-related change to the niche but also a great introduction to “normal” niche function — what biogerontologists might call the “pre-aged” case:

Stem Cells and Niches: Mechanisms That Promote Stem Cell Maintenance throughout Life

Niches are local tissue microenvironments that maintain and regulate stem cells. Long-predicted from mammalian studies, these structures have recently been characterized within several invertebrate tissues using methods that reliably identify individual stem cells and their functional requirements. Although similar single-cell resolution has usually not been achieved in mammalian tissues, principles likely to govern the behavior of niches in diverse organisms are emerging. Considerable progress has been made in elucidating how the microenvironment promotes stem cell maintenance. Mechanisms of stem cell maintenance are key to the regulation of homeostasis and likely contribute to aging and tumorigenesis when altered during adulthood.

Suckers for punishment will be pleased to see, in the same issue of Cell, another stem cell-related review, this one by Rossi et al. on the relationship between stem cell decline and age-related pathologies, including cancer.