J. Cell Biol. 187, 513–524 (2009)

When stem cells in the rat testes are at a developmental crossroads, they are able to make their decision independently of their surroundings, according to Zhuoru Wu and her colleagues at the University of Texas Southwestern Medical Center in Dallas.

Progeny of spermatagonial stem cells have two choices: become stem cells or differentiate into sperm. Some models predict that the cells' decision is determined by environmental cues.

But the researchers found that stem cells grown in the same culture medium gave rise to both differentiated cells and more stem cells. Mathematical modelling showed that each cell's decision was biased towards the stem-cell fate 67% of the time.