Figure 3. The example of grafts, which did not survive due to damage done to retinal pigment epithelium/choroid layers. A: This is a three-week-old subretinal graft with a typical neural retinal bulge but no human nuclei –positive (HNu [+]) cells. “V” is the vitreous space. The arrow points to the damage done to retinal pigment epithelium/choroid tissue (RPE/Ch), which likely causes the rupture of the blood-retinal barrier and exposes the xenogenic graft to the host’s immune system. B is the enlarged area shown in A, where RPE/choroid tissue is damaged (arrow). C: This is a three-week-old human embryonic stem cell-derived retinal progenitor cell graft with only few surviving HNu [+] cells left (white arrowheads); the RPE/choroid tissue is also damaged (white arrow). Panels D-E show the immunohistochemistry data done on retinal sections carrying the graft displayed in panel A. In panel D we demonstrate the accumulation of glial fibrillary acidic protein (GFAP), an early indicator of retinal distress, in the host retina. In panel E we show that microglia/macrophage marker ionized calcium binding adaptor molecule 1 (Iba-1) (known to be upregulated during the activation of these immune cells) is heavily present inside the graft and in the host retina around the grafted area. The asterisk shows the area in the main image, enlarged in the inset. The inset depicts several Iba-1 - positive cells in the host neural retina, with a typical microglial morphology. The scale bar used in panels C-E is 100 μm. F: This is a surviving 3-week-old subretinal graft shown for comparison; note the GFAP activation in the host retina above the graft, which did not affect the survival of such graft. The scale bar used in panel F is 50 μm. Abbreviations used in this legend are the following: ONL, outer nuclear layer; INL, inner nuclear layer; RGC, retinal ganglion cell (layer).