Wnt signaling has different temporal roles during retinal development.

Differentiation of neural retinal precursor (NRP) cells in vertebrates follows an established order of cell-fate determination associated with exit from the cell cycle. Wnt signaling regulates cell cycle in colon carcinoma cells and has been implicated in different aspects of retinal development in various species. To better understand the biological roles of Wnt in the developing retina, we have used a transgenic and pharmacological approach to manipulate the Wnt signaling pathway during retinal development in medaka embryos. With the use of both approaches, we observed that during the early phase of retinal development Wnt signaling regulated cell cycle progression, proliferation, apoptosis, and differentiation of NRP cells. However, during later phases of retinal development, proliferation and apoptosis were not affected by manipulation of Wnt signaling. Instead, Wnt regulated Vsx1 expression, but not the expression of other retinal cell markers tested. Thus, the response of NRP cells to Wnt signaling is stage-dependent. (c) 2009 Wiley-Liss, Inc.