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The irises of human eyes exhibit a wide spectrum of colours. Eye color is a polygenic phenotypic trait determined by two factors: the pigmentation of the eye's iris [1] [2] and the frequency-dependence of the scattering of light by the turbid medium in the stroma of the iris.
Despite the wide range of colors, the only pigment that contributes substantially to normal iris color is the dark pigment melanin. The quantity of melanin pigment in the iris is one factor in determining the phenotypic eye color of an organism. Structurally, this huge molecule is only slightly different from its equivalent found in skin and ...
The color of the mammalian, including human, iris is very variable. However, there are only two pigments present, eumelanin and pheomelanin . The overall concentration of these pigments, the ratio between them, variation in the distribution of pigment in the layers of the stroma of the iris and the effects of light scattering all play a part in ...
The green color is caused by the combination of: 1) an amber or light brown pigmentation in the stroma of the iris (which has a low or moderate concentration of melanin) with: 2) a blue shade created by the Rayleigh scattering of reflected light. [36] Green eyes contain the yellowish pigment lipochrome. [56]
Contributing to vision, the retina contains the photosensitive rod and cone cells and associated neurons. To maximise vision and light absorption, the retina is a relatively smooth (but curved) layer. It has two points at which it is different; the fovea and optic disc. The fovea is a dip in the retina directly opposite the lens, which is ...
The inner layer of the optic cup is made of neuroepithelium (neural retina), while the outer layer is composed of retinal pigment epithelium (RPE). Experiments have determined that RPE cell differentiation and maintenance requires interaction with neighboring tissues, most likely canonical Wnt signaling, while neural retina differentiation is ...
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Being color blind can change this, and there have been some verified reports of people with four types of cones, giving them tetrachromatic vision. [ 4 ] [ 5 ] [ 6 ] The three pigments responsible for detecting light have been shown to vary in their exact chemical composition due to genetic mutation ; different individuals will have cones with ...