Figure 8.

Ptf1a expression is regulated by negative extrinsic feedback from inhibitory cells. Extrinsic signals influence the expression of Ptf1a:GFP and generation of inhibitory subtypes. Tg(ptf1a:GFP) were injected with H2B:RFP to mark all nuclei red. A few cells were then transplanted into host embryos that were either WT or Ptf1a MO2 morphants. A, B, Micrographs showing examples of transplanted cells in the wild-type (A) or morphant embryos (B). Superimposed channels shown in first column, followed by red and green channels alone. Ptf1a:GFP-expressing cells were consistently found in layers corresponding to horizontal and amacrine cells. D, Quantification of percentage of transplanted cells that express Ptf1a:GFP cells and generate inhibitory neurons. The proportion of transplanted cells (red) that turned on Ptf1a:GFP expression (green) was significantly higher in the MO background, especially in retinas containing fewer transplanted cells. Numbers in bars indicate the number of analyzed cells arising from 20 (WT), 5 (large clones), or 29 (small clones) embryos. **p < 0.01, ***p < 0.001. D–F, Subtype composition of transplanted clones. Antibodies against calretinin, parvalbumin, or calbindin were used to determine the proportion of transplanted cells that would generate these subpopulations in WT versus morphant environments. ONL, Outer nuclear layer; OPL, outer plexiform layer; INL, inner nuclear layer; IPL, inner plexiform layer; GCL, ganglion cell layer. Scale bars: (in B) A, B, 50 μm; (in F) D–F, 10 μm.