ABSTRACT
Dorsal retinal fate is established early in eye development, via expression of spatially restricted dorsal-specific transcription factors in the optic vesicle; yet the events leading to initiation of dorsal fate are not clear. We hypothesized that induction of dorsal fate would require an extraocular signal arising from a neighboring tissue to pattern the prospective dorsal retina, however no such signal has been identified. We used the zebrafish embryo to determine the source, timing, and identity of the dorsal retina-inducing signal. Extensive cell movements occur during zebrafish optic vesicle morphogenesis, however the location of prospective dorsal cells within the early optic vesicle and their spatial relationship to early dorsal markers is currently unknown. Our mRNA expression and fate mapping analyses demonstrate that the dorsolateral optic vesicle is the earliest region to express dorsal specific markers, and cells from this domain contribute to the dorsal retinal pole at 24 hpf. We show that three bmp genes marking dorsal retina at 25 hpf are also expressed extraocularly before retinal patterning begins. We identified gdf6a as a dorsal initiation signal acting from the extraocular non-neural ectoderm during optic vesicle evagination. We find that bmp2b is involved in dorsal retina initiation, acting upstream of gdf6a. Together, this work has identified the nature and source of extraocular signals required to pattern the dorsal retina.
Subject(s)
Bone Morphogenetic Protein 2/metabolism , Ectoderm/physiology , Eye/embryology , Gene Expression Regulation, Developmental/physiology , Growth Differentiation Factor 6/metabolism , Morphogenesis/physiology , Retina/embryology , Zebrafish Proteins/metabolism , Zebrafish/embryology , Animals , Cell Differentiation/physiology , DNA Primers/genetics , Ectoderm/metabolism , Gene Expression Regulation, Developmental/genetics , Genotype , In Situ Hybridization , Polymerase Chain Reaction , Pyrazoles , Pyrimidines , Retina/cytology , Zebrafish/geneticsABSTRACT
Accurate retinotectal axon pathfinding depends upon the correct establishment of dorsal-ventral retinal polarity. We show that dorsal retinal gene expression is regulated by Wnt signaling in the dorsal retinal pigment epithelium (RPE). We find that a Wnt reporter transgene and Wnt pathway components are expressed in the dorsal RPE beginning at 14-16 hours post-fertilization. In the absence of Wnt signaling, tbx5 and Bmp genes initiate normal dorsal retinal expression but are not maintained. The expression of these genes is rescued by the downstream activation of Wnt signaling, and tbx5 is rescued by Bmp signaling. Furthermore, activation of Wnt signaling cannot rescue tbx5 in the absence of Bmp signaling, suggesting that Wnt signaling maintains dorsal retinal gene expression by regulating Bmp signaling. We present a model in which dorsal RPE-derived Wnt activity maintains the expression of Bmp ligands in the dorsal retina, thus coordinating the patterning of these two ocular tissues.
