TESTES AS MODELS FOR CELLULAR PLASTICITY
The Drosophila testis has proven to be an excellent model system to study the biology of stem cells and their microenvironments due to the relative simplicity of this tissue and an unparalleled collection of genetic tools available to probe it functionally.
Similarly, the vast amount of genetic tools available in mice allows us to study somatic cell regeneration in the mouse testis.
DROSOPHILA: TWO STEM CELLS, ONE NICHE
The Drosophila testis niche contains two stem cell populations: germ line stem cells and somatic cyst stem cells. Cyst stem cells wrap around germline stem cells and attach to a cluster of non-dividing somatic cells called the hub. The stem cells receive signals from the hub that promote their stem cell character. When stem cells divide, their daughter cells are pushed out of the niche, away from the hub. The subsequent reduction in signaling from the hub causes the daughters to differentiate.
MOUSE: REIMAGINING THE STEM CELL NICHE
The mouse testis contains seminiferous tubules that house the germ cells, differentiating spermatocytes and mature sperm. Outside of these tubules is the interstitial space. Macrophages and blood vessels can be found in the interstitial space, along with Leydig cells. Although they are in the interstitium, Leydig cells provide testosterone required for proper spermatogonial stem cell differentiation within the tubular stem cell niche.