A direct and melanopsin-dependent fetal light response regulates mouse eye development

During retinal vascular development there is simultaneous regression of the hyaloid vasculature and formation of the retinal vasculature; here it is demonstrated that regression of developing vasculature is light dependent and acts via the photoreceptor melanopsin. In the developing eye, as the retinal vasculature forms the blood vessels supplying the transparent hyaloid membrane surrounding the vitreous body regress. This process is clinically important as excessive vascular growth is a major cause of blindness in premature infants. Surprisingly, this study reports that light is involved in this major change in tissue architecture. Richard Lang and colleagues find that light stimulates regression of developing hyaloid vasculature in mice, acting through the photoreceptor melanopsin. In the absence of either light or melanopsin, vascular endothelial growth factor A is upregulated and abnormal retinal angiogenesis results. Whether the same processes are involved in human eye development remains to be determined. Vascular patterning is critical for organ function. In the eye, there is simultaneous regression of embryonic hyaloid vasculature1 (important to clear the optical path) and formation of the retinal vasculature2 (important for the high metabolic demands of retinal neurons). These events occur postnatally in the mouse. Here we have identified a light-response pathway that regulates both processes. We show that when mice are mutated in the gene (Opn4) for the atypical opsin melanopsin3,4,5, or are dark-reared from late gestation, the hyaloid vessels are persistent at 8 days post-partum and the retinal vasculature overgrows. We provide evidence that these vascular anomalies are explained by a light-response pathway that suppresses retinal neuron number, limits hypoxia and, as a consequence, holds local expression of vascular endothelial growth factor (VEGFA) in check. We also show that the light response for this pathway occurs in late gestation at about embryonic day 16 and requires the photopigment in the fetus and not the mother. Measurements show that visceral cavity photon flux is probably sufficient to activate melanopsin-expressing retinal ganglion cells in the mouse fetus. These data thus show that light—the stimulus for function of the mature eye—is also critical in preparing the eye for vision by regulating retinal neuron number and initiating a series of events that ultimately pattern the ocular blood vessels.