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Figure 2. Expression patterns of XLhx2 during eye formation in whole-mount (A,C,D) and sectioned (B,E-I) Xenopus embryos. A,B: XLhx2 is expressed in the eye field of the stage 12.5 embryo. Arrow shows expression is limited to the neuroectoderm. C,D: Anterior views show how the XLhx2 expression pattern resolves from a single eye field (stage 15) into the two eye primordia (stage 18). E: Evaginating optic vesicles and ventral forebrain stain throughout for XLhx2. Arrow and arrowhead point to presumptive retinal pigment epithelium (PRPE), optic stalk (OS), and presumptive neural retina (PNR), respectively. F: Dashed line outlines PRPE and PRN while arrow points to developing lens placode, where XLhx2 expression was undetectable. G: Neural retina (NR) expresses XLhx2, but lens vesicle (LV) does not. Arrows point to the peripheral retina where Lhx2 levels appear elevated. H: At stage 35/36, XLhx2 expression is reduced in the (a) presumptive outer nuclear and (b) ganglion cell layers. I: At stage 41, XLhx2 expression is limited to a subset of cells in the inner nuclear layer (INL) and ciliary marginal zone (CMZ, arrow and outlined in red). In G-I, the black dashed line denotes the border between neural retina and lens (Le). Dorsal is top in all panels. J-L: Relative expression patterns of the retinal transcription factors Lhx2, Pax6, Six3, Rx1, and Optx2 in the stage 41 Xenopus eye. J: Lhx2 expression is shown in light blue. K: Pax6, Six3, and Pax6 + Six3 expression is illustrated in blue, red, and violet, respectively. L: Rx1, Optx2, and Rx1 + Optx2 expression is illustrated in pink, yellow, and orange, respectively. At stage 41, the retina forms three nuclear layers. The outer nuclear layer (ONL - Rx1+) consists of rod and cone photoreceptors. The inner nuclear layer (INL) is subdivided into inner and outer regions. The outer INL (Rx1+; Optx2+; Six3+) contains mostly horizontal and bipolar cells while the inner INL (Lhx2+; Pax6+; Optx2+) contains mostly amacrine, Müller, and a few bipolar cells. The retinal ganglion cell layer (GCL) expresses Pax6, Optx2, and Six3. See the text for additional details of the expression patterns at these and earlier developmental stages. |
