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The embryonic progenitors that give rise to the vertebrate retina acquire their cell fate identity through a series of transitions that ultimately determine their final, differentiated retinal cell fates. In Xenopus, these transitions have been broadly defined as competence, specification, and determination. The expression of several transcription factors within the anterior neural plate at the time when the presumptive eye field separates from other neural derivatives suggests that these genes function to specify competent embryonic progenitors toward a retinal fate. In support of this, we demonstrate that some transcription factors expressed in the anteriorneural ectoderm and/or presumptive eye field (otx2, pax6, and rx1) change the fate of competent, ventral progenitors, which normally do not contribute to the retina, from an epidermal to a retinal fate. Furthermore, the expression of these factors changes the morphogenetic movements of progenitors during gastrulation, causing ventral cells to populate the native anterior neural plate. In addition, we experimentally demonstrate that the efficacy of pax6 to specify retinal cells depends on the position of the affected cell relative to the field of neural induction. Thereby, otx2, pax6, and rx1 mediate early steps of retinal specification, including the regulation of morphogenetic cell movements, that are dependent on the level of neural-inductive signaling.
FIG. 3.
Copyright 2001 by Academic Press. All rights of reproduction in any form reserved.
Transverse sections of tadpole retinas in which the V1.1.1 blastomere was injected with mRNA encoding otx2, rx1, or pax6 mixed with GFP mRNA as a lineage tracer. GFP-expressing cells are found in the retina (r) and span all the cellular layers (white arrowheads). The number of GFP-expressing cells varied from a single cell (D) to small blocks of cells (A) and large, coherent clones (B, C). le, lens.
FIG. 4. Ectopic retinal structures form after expression of pax6 in ventral blastomeres. (A) Transverse sections through stage-44 to -45 embryos in which pax6 was injected into the V1.2.1 blastomere. (A) DIC image of an inverted eye-like structure near the otic vesicle. Le, lens-like tissue; arrow, RPE-like tissue. (B) Extension of the retinal pigmented epithelium (left arrow) toward the brain and displaced photoreceptor (PH) cells within a pax6/GFP-expressing clone within the retina. (C) Ectopic photoreceptor cells (arrows) in an ectopic eye-like structure. (D) Retina appears duplicated, as indicated by the double-cup arrangement of photoreceptors (red) associated with an enlarged lens (le) that is expressing pax6 (green). (E, F) Transverse sections through stage-44 to -45 embryos in which pax6 and noggin mRNA were coinjected into the V1.1.1 blastomere. (E) Ectopic photoreceptor (PH) located in a circular, GFP-positive structure near the heart (ht). (F) Ectopic site of RPE (arrow) near a normal retina. Injection of noggin alone into V1.1.1 does not cause the formation of ectopic retinal tissue (Table 1).
FIG. 5. The formation of retinal phenotypes by ventralpax6 expression is dependent on a blastomere position in the field of neural induction. (A) Lateral view of tadpole in which the V1.2.1 blastomere was injected with GFP alone, demonstrating the normal distribution of this clone in the headepidermis. ey, eye; gt, gut. (B, C) Lateral views of two tadpoles in which V1.2.1 was coinjected with pax6 and GFP mRNAs. (B) A large ventral mass (arrow) is associated with RPE-like tissue (arrowheads) that extends towards the brain. Tissue section analyses showed that the ventral mass contained ectopic eye structures and the native retina contained GFP-expressing cells (Table 1). (C) Ectopic RPE-like tissue (arrowheads) extends from the retina towards the brain (arrowheads). In addition, the retina is enlarged, irregular in shape, and pax6-expressing cells (green) stream posteriorly from the lens (arrow). (D) The same tadpole in which the V1.1.2 blastomere was coinjected with pax6 and GFP mRNAs. Injected cells (green) populate the trunkepidermis (D), consistent with the normal fate map (Moody, 1987). There is no evidence of ectopic, pax6-expressing retinal structures in the head (E), as observed when V1.2.1 was injected. Furthermore, the eye on the injected side (right side in E and F) looks normal and has no extensions of RPE-like tissue (F, arrowhead). Tissue section analyses, however, demonstrated GFP-labeled cells in the native retina in a small number of similar embryos (Table 1). (G) The same tadpole in which blastomere V1.2.1 was coinjected with pax6, bmp4, and GFP mRNAs. Injected cells (green) are more posterior (G) than controls (A), and are not in the retina (G). The retina on the injected side is normal in size (H, I) and there are no ectopic extensions of RPE-like tissue (I, arrowhead). Tissue section analyses confirmed that there were no GFP-labeled cells in the native retina, nor were there ectopic retinal structures (Table 1).
FIG. 6. Anterior views of neural plate (stage 156) embryos. Sox3 expression (blue) delineates the neural plate and cranial placodes (pl). Red cells are gal-positive descendants of blastomere V1.1.1 that also are expressing the indicated transcription factor. In gal-injected controls (ctrl), the most anterior cells of the clone are distant from the neural plate, whereas in transcription factor-injected embryos these cells overlap the anterior boundary of the neural plate. There are no ectopic patches of sox3 expression. The same results were obtained with a sox2 probe.
FIG. 7. otx2, pax6, and rx1 alter the positions of ventral blastomere clones during gastrulation. (A) Animal cap view of stage-11.5 control V1.1.1 clone. Cells, centered on the animal pole (ap), are coherent with interdigitation along the border. (B) Animal cap view of a stage-11.5 otx2-injected V1.1.1 clone. Some cells (arrows) are dispersed distant from the coherent members of the clone, which are centered on the animal pole (ap). (C) Ventral view of stage-11.5 pax6-injected V1.1.1 clone, showing dispersed cells that have invaded the marginal zone. Arrow depicts the equator. Animal pole (ap) is to the left. (D) Stage-11.5 rx1-injected clone. Same as in (C). (E) Animal cap view of a stage-12 control V1.1.1 clone, which is retreating from the animal pole (ap). (F) Animal cap view of a stage-12 otx2-injected V1.1.1 clone that is more dispersed and remains predominantly at the animal pole (ap). (G) Animal cap view of stage-12 pax6-injected V1.1.1 clone is similar to (F). (H) Animal cap view of a stage-12 rx1-injected V1.1.1 clone is similar to (F). (I) Anterior view of a stage-13 control V1.1.1 clone. The anterior-most cells are distant from the anterior border (indicated by red line) of the presumptive neural plate (np). Dorsal is to the top. (J) The stage-13 otx2-injected V1.1.1 clone overlaps the anterior border (red line) of the presumptive neural plate (np). Same orientation as in (I). (K) Stage-13 pax6-injected V1.1.1 clone, as in (J). (L) Stage-13 rx1-injected V1.1.1 clone, as in (J). (M) Ventral view of a stage-12 control V1.1.2 clone that has retreated from the animal pole (ap) and is mostly in the vegetal half of the embryo. yp, location of yolk plug. (N) Ventral view of a stage-12 otx2-injected V1.1.2 clone. Cells are more dispersed and many remain at the animal pole (ap). Cells do not reach the vegetal yolk plug (yp). (O) Ventral view of a stage-12 pax6-injected V1.1.2 clone, as in (N). (P) Anterior view of a stage-13 rx1-injected V1.1.2 clone, which overlaps the anterior rim (red line) of the presumptive neural plate (np). Dorsal is to the top.
