Figure 5

Neurogenesis in the retina and tectum of E. coqui ( A - C, G-I ) and X. laevis ( D-F, J-L ) as revealed by in situ hybridization for NeuroD in transverse paraffine sections. A-F: At early embryonic stages, NeuroD begins to be expressed in scattered cells throughout the retina in both species (A, D). Subsequently, most retinal cells express NeuroD, until NeuroD is downregulated in the central, inner part of the retina (asterisks) at midembryonic stages of both species (B, E). The retina of E. coqui has grown to much larger size at this stage than the retina of X. laevis (note different magnification). At later stages, NeuroD expression is restricted to the ciliary margin (arrowheads), the outer part of the inner nuclear layer and the outer nuclear layer in both species (C,F) and to scattered cells in the retinal ganglion cell layer in E. coqui. The inner and outer plexiform layers (black and red arrows, respectively) are much thinner in E. coqui than in X. laevis. G-L: In contrast to X. laevis (J-L), in which the tectum (arrows) expresses NeuroD from stage NF 46 on, the tectum of E. coqui never shows strong NeuroD expression at any stage (G-I) although expression is evident in other parts of the midbrain similar to X. laevis (asterisks). Abbreviations: INL: inner nuclear layer; IPL: inner plexiform layer; ONL: outer nuclear layer; OPL: outer plexiform layer; PR: layer of photoreceptor outer segments; RGC: retinal ganglion cell layer. Bar: 100 μm in all panels.