Structures called centrosomes help to organize networks of microtubules (straight lines) inside cells. (A) Early in the cell cycle (during interphase; left column) the mother centrosome (green) in a fruit fly male germline stem cell is slightly more active than the other centrosome (red), which helps to anchor (brown) the stem cell to the hub cell (pale purple). Later in the cell cycle (during metaphase and telophase) both centrosomes are equally active, organizing the microtubules to form a symmetrical spindle apparatus across the center of the cell. When the cell divides it produces a new germline stem cell and a gonialblast that are equal in size. (B) The mother centrosome in a Klp10A depleted germline stem cell is more active than the other centrosome in interphase, metaphase and telophase, which leads to an asymmetric spindle and siblings of different size. The larger cell remains attached to the hub cell, adopting stem cell fate, whereas the smaller sibling often dies. (C) Neural stem cells in fruit flies are intrinsically polarized (green and red crescent, respectively) and also contain asymmetric centrosomes, which result in an asymmetric spindle at telophase. The larger cell is destined to become the neural stem cell whereas the smaller cell becomes a ganglion mother cell. In contrast to germline stem cells, it is the asymmetric localization of motor proteins in neural stem cells during anaphase (between metaphase and telophase; not shown) that is largely responsible for producing sibling cells that are unequal in size. Small circles in interphase cells represent cell nuclei; the chromosomes are not shown.