The steady production of sperm relies on the number of sperm stem cells in the testis remaining constant. Researchers including Assistant Professor Yu Kitadate and Professor Shosei Yoshida (developmental biologists at the National Institute for Basic Biology within the National Institutes of Natural Sciences in Japan) and Professor Benjamin Simons (a theoretical physicist at the University of Cambridge in the U.K.) have revealed a novel mechanism for stem cell population control. Their results show that constant sperm stem cell numbers are achieved in mouse testes through a self-organized process in which they actively migrate and compete for a limited supply of self-renewal-promoting fibroblast growth factors (FGFs). This study was published on line in Cell Stem Cell on Nov. 20th, 2018.