Fig. 1. Fgf2 induces retina regeneration in two distinct ways. (A) At E4 the retina is removed surgically leaving behind RPE and CB/CMZ. An Fgf2-soaked heparin bead (*) is then placed in the eye cup. (B) A heparin bead (not visible in this section) alone does not cause regeneration after 7 days (E11). (C) By E11, Fgf2 induces regeneration from the CB/CMZ (cr) and by the transdifferentiation of the RPE (td). (D-O) Histology of normally developing as well as regenerating retina at E7 (D-F), E9 (G-I), E11 (J-L) and E15 (M-O). D,G,J,M show normal development at each stage. Three days after retinectomy (E7) a transdifferentiated neuroepithelium (E) as well as a neuroepithelium generated from cells in the CB/CMZ (F) are present. At E9, the transdifferentiated neuroepithelium (H) as well as the neuroepithelium arising from the CB/CMZ (I) thicken and grow. (K,L) Seven days post-retinectomy, at E11, the various retinal layers become visible. By E15, regenerated retinas (N,O) are laminated and resemble an E11 developing retina (J). Scale bar: 100 µm (A,B); 500 µm (C); 100 µm (D-O). GCL, ganglion cell layer; IPL, inner plexiform layer; INL, inner nuclear layer; OPL, outer plexiform layer; INL, inner nuclear layer; ne, neuroepithelium; l, lens; CB/CMZ, ciliary body/ciliary marginal zone; RPE, retinal pigmented epithelium.