Developmental Biology Select

This Developmental Biology Select discusses recent progress toward understanding the regulation of stem cell division and the impact of stem cell self-renewal on tissue homeostasis. New findings include the discovery that stem cell division is diminished by misorientation of centrosomes during aging. Other recent work sheds light on the effects of stress signaling on stem cell proliferation, reveals the presence of an actively cycling population of hair follicle stem cells, defines factors that maintain hematopoietic stem cell quiescence, and provides insight into the formation of lateral roots in plants. The centrosomes of germ stem cells (GSCs) of the male fruit fly are typically oriented perpendicular to the hub cells that comprise the stem cell niche. During asymmetric division of the GSC, the daughter that remains in contact with the hub cell retains stem cell properties, whereas the daughter that lacks contact with the niche gives rise to the transit-amplifying cells whose progeny ultimately give rise to mature sperm. Recent findings by Cheng et al. (2008) show that centrosome misorientation is a major factor contributing to the decline in spermato-genesis observed during aging in the fruit fly Drosophila. They show that the frequency of GSCs with misoriented centrosomes steadily increases as flies age. Although these misoriented GSCs retain the ability to divide, they do so less frequently than GSCs with correctly oriented centrosomes. The authors also provide evidence that misoriented GSCs first need to reestab-lish the correct orientation before mitosis will occur. Hence, the net effect of misorientation is a decrease in GSC division that leads to a failure to maintain tissue homeostasis. Interestingly, one source of misoriented GSCs appears to be spermatogonia that dedifferentiate to occupy a vacant stem cell niche. Although dedifferentiated cells serve to replenish the stem cell pool, it appears that their effectiveness in restoring youthful spermatogenesis is impaired by an elevated propensity for centrosome misorientation. Future work may delve further into the cell-intrinsic factors that lead to centrosome misorientation or could explore whether changes in the niche are a major contributing factor. Given that stem cell self-renewal and proliferation in many tissues relies upon asymmetric cell division, this discovery may have widespread relevance, in particular for stem cells that reside in niches with a defined polarity. It is often said that stress takes a toll on the body. In their new work, Biteau et al. (2008) provide insight into the mechanisms by which stress signaling impacts …