Ectopic proliferation and Sox2 expressing cells in young adult Tg(flk1:RFP)is18 dysplastic retina. (a–e) Wild-type 4-month-old adult (n = 3) shows that PCNA positive cells are restricted to stem/progenitor cells at the ciliary marginal zone (cmz, arrow, c) and rod precursor cells in the photoreceptor outer nuclear layer (arrowheads, e). Sox2-expressing cells overlap with PCNA positive stem cells at the ciliary marginal zone (bracket, c) and are present in amacrine/displaced amacrine cells in the inner nuclear and ganglion cell layers (c, e). (f–j) Retina from 4-month-old Tg(flk1:RFP)is18 adult (n = 3) shows numerous regions of proliferation distributed throughout the neural retina (f). The ciliary marginal zone (g, h) and ventral ciliary circumferential artery (asterisk, g) are expanded. Small masses of PCNA and Sox2 positive cells (arrowheads, g, h) are present in the normally single-cell layered nonpigmented epithelium extending from the ciliary marginal zone (small arrow, g, h–compare to small arrow in wild type b). Ectopic regions of proliferation in the neural retina contain disorganized cells expressing PCNA and Sox2 (arrowheads, j). cmz, ciliary marginal zone; gcl, ganglion cell layer; inl, inner nuclear layer; onl, outer nuclear layer; pe, pigmented epithelium. Scale bars (a, f) 200 μm; all other panels 50 μm.