The choroidal vasculature is essential for normal retinal function. However, mechanisms that control choroid development are unknown. In the present study, we provide evidence that the retinal pigment epithelium (RPE) plays an essential role in regulation of the choroid development in the mouse eye. Transgenic mice that transiently express FGF9 in the embryonic RPE were generated. Postnatal eyes were analyzed by histology and in situ hybridizations. In the transgenic mice, most of the RPE was

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... nverted to neural retina. The choroid formed only in regions where patches of RPE were present. The choroid failed to develop in the absence of the RPE. The presence of the RPE appears to be required for choroid development, suggesting that molecular interactions between RPE and periocular mesenchyme are essential for melanocyte differentiation and vascular development in the choroid.