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???displayArticle.abstract??? Bone morphogenetic proteins (BMPs) act repeatedly in the development of nervous system tissues. While BMP signaling is critical for the early growth and patterning of the eye, we are interested in possible later functions of BMPs in the morphological development of retinal neurons and formation of synaptic connections. Therefore, we conducted an in situ hybridization analysis of the mRNA expression for the ligands Bmp2, -4 and 7 and the type Ia, Ib and II receptors (BmprIa, BmprIb and BmprII) during development of the retina of Xenopus laevis. Bmp4 mRNA is expressed in the dorsal retina and Bmp7 in the distalperipheral retina during the period of cell differentiation, while Bmp2 is not present in the eye. The type I receptors are expressed predominantly ventrally, from the optic vesicle stage until at least stage 35/36, after most cells have differentiated and many synaptic connections have formed. BmprII mRNA, however, is distributed evenly across the dorsoventral axis, with highest expression in retinal ganglion cell and inner nuclear layers.
Fig. 1. Bmp4 mRNA, but not Bmp2, is expressed in the forming Xenopus retina. (A,B) Spatial expression of Bmp2 mRNA in Xenopus embryos processed for wholemount in situ hybridization using digoxige- nin-labeled riboprobe. Lateral views of stage 26/28 (A) and stage 32 (B) embryos. (C,D) Expression of Bmp4 mRNA in wholemount embryos at stages 26/28 (C) and 32 (D). (E,F,H) Transverse sections through retinas of embryos processed for Bmp4 in situ hybridization at stages 26/28 (E), 32 (F) and 35/36 (H). (G) Immunolabeling of BrdU that has incorporated into dividing cells at stage 33/34 shows the location of the ciliary marginal zone (CMZ; white arrowheads), a proliferative region adjacent to the lens. Compare with Bmp4 expression (black arrowhead) at stage 32 (F). Anterior is to the left in panels (A-D). Scale bar in (E) is 50 μm for (E-H). Abbreviations: D, dorsal; V, ventral; ba, branchial arches; e, eye; h, heart; L, lens; nr, neural retina; p, pineal gland; ol, olfactory pits; ot, otic vesicle; rpe, retinal pigmented epithelium.
Fig.2. Expression of Bmp7 mRNA in the Xenopus retina during development. (A,B) Xenopus embryos processed for wholemount in situ hybridization with Bmp7 antisense riboprobe at stages 26/28 (A) and 32 (B). (C) Transverse section through the central portion of the optic vesicle of a stage 26/28 embryo labeled for Bmp7 mRNA (see line 1 in A). (D) Transverse section through the peripheral retina of stage 26/28 embryo (line 2 in A) reveals expression of Bmp7 mRNA along the entire dorsoventral axis. (E) Transverse section through a stage 32 embryo shows Bmp7 expression in the dorsal and ventral CMZ (black arrowheads). (F) A patch of Bmp7 expression (arrow) appears in the mesenchymeventral to the EF retina at stage 35/36. Anterior is to the left in panels (A,B). Scale bar in (C) for panels (C-F) is 50 μm. * in (C,D) shows the location of the interior of the optic vesicle prior to invagination of the optic cup. D, dorsal; V, ventral; ba, branchial arches; h, heart; L, lens; nr, neural retina; ot, otic vesicle; rpe, retinal pigment epithelium.
Fig. 3. Distribution of type I Bmp receptor transcripts in the developing Xenopus eye. (A) Wholemount in situ hybridization of a stage 28 embryo using a BmprIa antisense probe. (B-E) Transverse sections through the retinas of Xenopus embryos processed for BmprIa in situ hybridization, at stages 28 (B), 30 (C), 33/34 (D) and 35/36 (E). * in (C) is the forming RGCL. (F) Bmpr1b wholemount in situ hybridization of a stage 30 embryo. (G-J) Transverse retinal sections through stage 28 (G), 30 (H), 33/34 (I) and 35/36 (J) embryos show expression of BmprIb. Scale bar in (B) is 50 μm for (B-E) and (G-J). Abbreviations: D, dorsal; V, ventral; ba, branchial arches; e, eye; k, kidney; is, intersomitic region; L, lens; nr, neural retina; ot, otic vesicle; rpe, retinal pigment epithelium.
Fig. 4. Expression of BmprII mRNA in the developing Xenopus eye. (A) Stage 30 Xenopus embryo processed for BmprII in situ hybridization. BmprII transcripts are expressed in the eye (outlined by dotted line). (B,C, E,F) Ubiquitous expression of BmprII mRNA in the stage 28 retina (B) becomes refined to the inner layers of the neural retina and the lens at stage 30 (C), 33/34 (E) and 35/36 (F). Expression is absent from the emerging photoreceptors in the outer nuclear layer (onl). (D) A retina immunolabeled for Islet-1 shows the location of the RGCs in the inner- most layer of a stage 33/34 retina. Scale bar in (B) is 50 μm for panels (B- F). Abbreviations: D, dorsal; V, ventral; h, heart; L, lens; nr, neural retina; onl, outer nuclear layer; ot, otic vesicle; rgcl, retinal ganglion cell layer; rpe, retinal pigment epithelium.
