Secreted inducers in vertebrate eye development: more functions for old morphogens.

Cell signaling molecules secreted from strategically localized positions coordinate cell behavior to enable progressive specification of embryonic tissues. These molecules converge on a few signaling pathways that are reiteratively used in different tissues at different times for generating cell type-specific patterns of gene expression. Although our current knowledge of the system is fragmentary, eye development seems to follow this general strategy. In line with this idea, recent studies have added new information on how Fgf and Wnt signaling participates in the formation of the eye field. In addition, later on in development, Fgf controls the onset of retinal neurogenesis and Shh and GDF11 control its feedback regulation.