Pointing in the right direction

Skin, intestinal epithelium, and sperm cell populations are maintained by adult stem cells that strike a delicate balance: always dividing to produce one stem and one cell able to differentiate. Drosophila testes germline stem cells (GSCs) undergoing mitosis always produce one GSC and one gonialblast that subsequently differentiates into a germ cell. This self-renewal process requires a specific microenvironment or niche. In Drosophila, this is provided by a cluster of somatic cells called "the hub," whose local signals maintain GSC identity, but how the niche enables GSCs to strike a balance of renewal and differentiation has been unclear. In the September 12 Science, Yukiko Yamashita and colleagues at Stanford University reveal that intracellular mechanisms involving the centrosome and adenomatous polyposis coli tumor suppressor protein (APC) orient the mitotic spindle perpendicular to the local niche, allowing proper GSC division to occur (Science, 301:1547-1550, September 12, 2003)....