The visual basis of the biological clock “sees” the light: intrinsically photosensitive retinal ganglion cells

Mammalian eyes mediate both image-forming and non-image-forming visual functions.Non-image-forming vision provides a measure of the ambient light for the purposes of synchronization of circadian clocks to light/dark cycles and regulation of pupil size, pineal melatonin production and other functions.Traditionally, people used to believe that the classical photoreceptors (rods and cones) regulate both image-forming and non-image-forming visual pathways.However, a small subset of retinal ganglion cells called intrinsically photosensitive RGCs (ipRGCs) has been identified to be a third type of mammalian photoreceptor and determined to be photosensitive, recently.The discovery of ipRGCs has allowed for rapid progress in the past decade toward understanding the non-image-forming visual system, especially about how the circadian clock complete the synchronization with the light/dark cycle.The anatomical and developmental characteristics of ipRGCs, as well as its biological functions and regulation were reviewed in this paper.