Endocrine Abstracts

Many adult organs harbor stem cells that are used to sustain homeostasis and to replenish damaged tissue following injury or disease. While the therapeutic potential of stem cells has been much discussed its practical use is still lacking. One major obstacle is the difficulty of re-introducing stem cells into their organs.

Adult stem cells reside within a special environment (niche), which participates in every aspect of stem cell behavior. Many of the biological principles that govern stem cell behavior within a living body were worked out in fruit flies. These principles suggest that stem cells cannot be considered separately from their niches. They form one unit.

Oogenesis in the fly depends on germ line stem cells (GSCs). GSCs divide continually to produce one daughter cell that remains a stem cell, while the other differentiates to form an egg. Niche cells secrete factors that maintain GSCs and control their rate of division. They also tether GSCs and induce asymmetric localization of proteins and organelles within GSCs.

Our data suggests that the stem cell unit (GSCs and their niche) develops in unison. Most importantly, while niche cells affect GSCs, the opposite is also true. Stem cells affect their own support cells. Germ cells and their somatic support cells communicate via a feedback loop. GSCs produce a factor required for somatic cell survival. When GSCs are missing, support cells die. The support cells, on the other hand, produce a factor repressing germ cell proliferation. This feedback loop controls tissue homeostasis and allows the ovary to control the number of stem cells it possesses.

The conservation of biological principles and molecular players between flies and mammals suggest that many of the lessons learned in flies are applicable to humans. The implications for the interdependence of stem cells and niche cells will be discussed.