Figure 1. A model for the genetic regulation of lens induction.

The solid arrows represent pathway relationships that have been defined genetically by molecular epistasis analysis or that have been implied through pathway up- or down-regulation. For example, Prox1 expression is lost in mice with a placodal loss of Pax6 and so Prox1 is placed downstream. The dashed arrows represent direct physical interactions that have been defined by biochemical experiments. There are elements of speculation in this model. For example, Bmp4 expression is lost in the optic vesicle of Frs2α^2F/2F mutant, but it is unclear whether this regulation occurs within the optic vesicle (the model shown) or is a consequence of Frs2α-dependent signaling to the presumptive retina from the presumptive lens. See the text for further explanation of these interactions. The word “ligands?” placed between the boxes representing the presumptive lens and presumptive retina is designed to indicate that so far, no conventional FGF ligands are known to be critical for this inductive interaction. On the left side of the figure, the approximate embryonic (E) stages at which these events occur are indicated.