FIGURE 21-30 Two ways that stem cells can be induced to divide asymmetrically. (a) In response to an external cue, the cell polarizes, and fate determinants (red dots) become segregated, before cell division. Division then produces one stem cell and one differentiating cell. (b) Stem cells interacting with a stem-cell niche (red curved object) orient their mitotic spindle to give rise to a stem cell associated with the niche and a differentiating cell displaced from it. See S. J. Morrison and J. Kimble, 2006, Nature 441:1068.