Retinoic acid signaling within a subpopulation of rod photoreceptors in response to prolonged retinoic acid treatment. (A to C) Embryos carrying the RARE-YFP transgene were treated with DMSO (A to C) or 0.3 μM RA (D to F) from 36 to 60 hpf, and were processed as 4 um cryosections for indirect immunofluorescence with an anti-GFP antibody (green) and antibody to rod opsin (1D1; red). (A) Rod photoreceptors are found densely populated in a ventral patch (arrow) and more widely distributed in the dorsal retina. (B) In control embryos, endogenous reporter expression is limited to cells of the ventral retina, but found in all retinal layers in that region (bracket). (C) Merged panel from A and B showing some colocalization of YFP and rod opsin (arrowheads) in the ventral retina, and none in the dorsal retina (inset). (D) Rod photoreceptors in a retina from an embryo treated with RA. Arrow indicates the ventral retina. (E) In experimental embryos the RA treatment leads to widespread expression of YFP, including the ONL. (F) Merged panel from D and E, showing rods in the dorsal retina colocalized with YFP (arrowheads) as well as those not expressing YFP (arrow). Bar = 50 μm.