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Fig. 1. Schematic illustration of vertebrate lens development, relationships between inductive interactions and sequential activation of transcription factors. (A–G) Xenopus was chosen as the vertebrate representative since its developmental lineages of lens, retina and other related tissues have been well studied ( [Eagleson and Harris, 1990] and [Eagleson et al., 1995]). The developmental stages are indicated at the top of the figure. (A) Dorsal view of a neural plate-stage embryo (anterior at the top of the image) indicating the preplacodal ectoderm (PPE, purple), the presumptive lens ectoderm (PLE, dotted light blue), the neural plate (NP, light orange), and the presumptive retina field (PR, dark orange). (B) Transverse section of a neural plate-stage embryo through the PLE and PR. The red line in A indicates the plane of the section. Lens and retina lineages are shown in light blue and dark orange, respectively, in B-G, and mesoderm and endoderm (ME) are shown in light green. (C) Transverse section of a neural tube-stage embryo where the optic vesicle (OV), which develops from the PR, reaches the PLE. (D-F) Close-ups showing lens vesicle formation. The PLE overlying the OV becomes thickened to form the lens placode (LP), which subsequently separates from the head ectoderm to form the lens vesicle (LV). OC, optic cup. (G) Close-up of a maturating lens. LE, lens epithelium; LF, lens fiber; NR, neural retina. (H) Major signaling factors are indicated in red with dotted gray arrows. FGF signaling from the neural retina is responsible for the formation of the secondary lens fibers from the lens epithelium (Lovicu and McAvoy, 2005). The other signaling pathways are explained in the text. At the bottom, Expression profiles of preplacodal genes, lens-specification genes and lens-differentiation genes (pink, green, and blue arrows, respectively) studied in Xenopus, zebrafish, chicken and mouse embryos. The dotted blue lines of FoxE3 and Pitx3 represent species differences in initial expression profiles. Details and references are described in the text. (I) In situ hybridization analysis of Pax6 expression in a neural plate-stage Xenopus embryo (dorsal view). The PLE and PR are indicated. The boundary between the neural plate and PPE is indicated by a white broken line. Note that preplacodal Pax6 expression occurs broadly adjacent to the anterior margin of the neural plate. |
