Wnt signaling in retinal stem cells and regeneration.

In the vertebrate retina, stem cells with prolonged proliferative capacities reside in the most peripheral region, the ciliary marginal zone (CMZ), and they persist even after the functional eye has formed. These stem cells contribute to the formation of the retinal structures during the postnatal period in vivo, or can expand as neurospheres in vitro. Despite the wealth of anatomical descriptions of the characteristics of CMZ cells, molecular mechanisms for their specification or maintenance have long been uncharacterized. Recent studies provide evidence that certain secreted signaling molecules act as key regulators at multiple steps during these processes. In this review, we discuss the molecular basis for the regulation of retinal stem cells and their related cell types, especially focusing on the role of Wnt signaling.