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This study describes a whole embryo and embryonic field analysis of retinoic acid's (RA) effects upon Xenopus laevis forebrain development and differentiation. By using in situ and immunohistochemical analysis of pax6, Xbf1, and tyrosine hydroxylase (TH), gene expression during eye field, telencephalon field, and retinal development was followed with and without RA treatment. These studies indicated that RA has strong effects upon embryonic eye and telencephalon field development with greater effects upon the ventral development of these organ fields. The specification and determination of separate eye primordia occurred at stage-16 when the prechordal plate reaches its most anterior aspect in Xenopus laevis. Differentiation of the dopaminergic cells within the retina was also affected in a distinct dorsoventral pattern by RA treatment, and cell type differentiation in the absence of distinct retinal laminae was also observed. It was concluded that early RA treatments affected organ field patterning by suppression of the upstream elements required for organ field development, and RA's effects upon cellular differentiation occur downstream to these organ determinants' expression within a distinct dorsoventral pattern.
Figure 3. Whole embryo in situ hybridization for pax6 and Xbf1. A: A stage-15 embryo viewed frontally. The pax6-expression pattern is restricted to anteriorlateral plate areas (including the lateral neural ridge) and a small bridge along the most anterior neural plate. pax6 expression is first detected at stage 12.5. The small arrow indicates the anterior extent of Xbf1 expression. B: A stage-13/14 embryo viewed ventrofrontally. The Xbf1-expression pattern (indicated by the small arrow) is within the anterior neural ridgetissue and a small portion of the anterior neural plate. The most ventral areas of Xbf1 expression overlap with the most anterior borders of pax6 expression, especially along the medial neural ridge. C: A stage-29/30 embryo. The Xbf1-expression pattern includes the most anterior prospective telencephalon, the branchial neural crest, and the dorsoanterior portion of the optic vesicle (arrow). Scale bars = 250 mu m in A,C.
Figure 4. The effects of whole embryo immersion in retinoic aid upon Xbf1 expression. A: At stage 12, embryos were placed in retinoic acid (RA) (10-6 M) for 2 hr and allowed to develop to stage 28. There is very little positive Xbf1 staining. This Xbf1 staining is rather diffuse, and optic vesicles did not evaginate. Much of the staining was probably nonspecific staining due to the open anteriorbrain cavity. B: At stage 13, embryos were placed in RA (10-6 M) for 2 hr and allowed to develop to stage 28. There was a small amount of Xbf1 expression (arrow) in the most anterior aspect of the embryo (presumptive telencephalon). These embryos had optic vesicles, but the anterior neuropore did not fully close. C: Embryos incubated in RA (10-6 M) at stage 14 for 2 hr exhibited normal Xbf1-expression patterns after it was allowed to develop to stage 28. These embryos also had normal optic vesicles. Scale bar = 250 mu m in A (applies to A-C).
Figure 5. The effects of whole-embryo immersion in retinoic acid (RA) upon pax6 expression. A: Embryos were placed in RA (10-6 M) at stage 12 for 2 hr and allowed to develop to stage 28. There is very little positive pax-6 staining and optic vesicles did not evaginate. The staining was probably nonspecific staining within the open anteriorbrain cavity. B: Embryos placed in RA (10-6 M) at stage 14 for 2 hr exhibited little pax 6 expression, despite that these stage-28 embryos developed normal optic vesicles. C: Embryos placed in RA (10-6 M) at stage 16 for 2 hr exhibited pax 6 expression at stage 28, but this expression pattern differed from stage-28 untreated embryos (D) in that hindbrain expression of pax 6 is anteriorized to within the midbrain (double arrows) in RA-treated embryos compared with normals (D). Scale bar = 250 mu m in D (applies to A-D).
