Regulation of adult stem cells by Niche-sensing cellular protrusions

Adult stem cells maintain homeostatic tissue turnover or remain quiescent until tissue injury in a manner dependent on signaling cues from their niche. How stem cells physically interact with niche cells, and whether stem cells possess morphologies that optimize niche recognition and signal reception, is understudied. Here, we discuss several different adult stem cell types in Drosophila and mice that display distinctive morphologies, notably several types of cellular protrusions. Such protrusions function in multiple ways, including: (1) ensuring that the signaling range of ligands secreted by niche cells is restricted only to the stem cell; (2) allowing stem cells to interact simultaneously with multiple, spatially separated, niche cell types; and (3) acting as dynamic sensors of the niche. The in vivo morphology of many adult stem cell types is not well established, and it is likely that cellular protrusions are employed as a means of niche interaction by a variety of such stem cells.