Figure 8. Fgf signaling withdrawal dependent photoreceptor death triggers proliferation response in ONL and INL.

A) The control retina shows few BrdU+ cells in the ONL (arrowhead). B) Non-heatshocked control transgenic fish show similar numbers of BrdU+ nuclei in the ONL (arrowhead) as the control. C) After 3 days of heat shock treatment, a strong proliferation response is detectable in the INL (arrowhead). D) After five days, cell clusters and fusiform-shaped cells are found in the INL (arrowhead). E) After seven days of heat shock induction, a large number of BrdU+ nuclei are located in the ONL (arrowhead). F) Quantifications of BrdU+ cells per section. Under control conditions, control siblings and non-treated transgenic fish show hardly any BrdU incorporation. After one day of HS induction, proliferation increases in both groups of experimental fish compared to non-heatshocked controls. There is no significant difference between transgenic and WT siblings (p = 0,11). After three days, many cells proliferate mainly in the INL of transgenic siblings (p = 0,01). At seven days of Fgf signaling inhibition, the number of proliferating cells in the INL and ONL increases even further in dn-fgfr1 fish (p = 4,3E-13). Shown are the mean numbers of BrdU+ nuclei/section. The error bars indicate the SEM. p-values: *≤0.05, **≤0.01, ***≤0.001. G) After 5 d of HS, experimental fish were soaked in BrdU, followed by a one month chase time. In control siblings, BrdU+ nuclei were found after one month of regeneration in the ONL (arrowhead). H) In transgenic fish elongated BrdU+ cell nuclei, which are characteristic for photoreceptor cells, are detected in the photoreceptor layer (arrowhead). I) Zpr1 and BrdU (arrowhead) double positive nuclei were not detectable in control fish. J) In contrast, numerous Zpr1 and BrdU double-labeled cells were found in the photoreceptor layer in retinas of transgenic fish (arrowhead). Scale bars = 20 µm.