Asymmetric cell division is associated with control over spindle orientation in several models, such as the fly neuroblast and the worm P1 cell. In both of these cases, the spindle is reoriented during mitosis. But in fly germline stem cells (GSCs), which divide asymmetrically to produce one stem cell and one cell that initiates differentiation, Fuller's group now shows that the GSCs are oriented throughout the cell cycle, not just during mitosis.GSCs align themselves with the surface of their niche, known as the hub—a cluster of somatic cells in the testes that instruct neighboring GSCs to retain stem cell identity. Yamashita et al. show that GSCs build their spindle perpendicular to the hub by keeping one centrosome in close contact with the hub—even during interphase. After the cell divides, one daughter remains connected to the hub, and thus maintains stem cell identity, whereas the other daughter is displaced away and differentiates. Disruption of centrosome function interferes with this polarity. As a result, both daughter cells contact the hub, leading to an excess of GSCs.
The boundary between the hub and the GSC contained high levels of cadherins and a fly homologue of APC, which is thought to help orient spindles in epithelial cells. APC mutant GSCs have mispositioned centrosomes, misoriented spindles, and an excess of GSCs. Thus, the APC–cadherin complex may anchor one GSC centrosome near the hub, leaving the other free to roam. ▪