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Eye Color - Iris Structure

differences stroma light pigmented pigment

The iris is the most visible portion of the uveal tract, which is the middle compartment of the eye. The iris is made up of blood vessels and connective tissue, in addition to melanocytes and other pigmented cells that are responsible for its distinctive color.

The iris contains muscles that control its movement and allow for changes in the size of the pupil, controlling the amount of light that enters the eye and that ultimately reaches the retina. Though its function is easily observed and obvious to any who look at it under varying lighting conditions, it is the appearance of the iris—specifically, its color—that is most striking and apparent. The structure itself takes its name from Iris, the Greek goddess of the rainbow and messenger of the gods.

The iris consists of two layers of different embryological origin. The anterior border of the iris consists of the stroma, a loose and interrupted layer of connective tissue. It is composed of melanocytes and nonpigmented cells, as well as other types of cells and tissues. Melanocytes contain melanin, a brown or black pigment. The overall structure of the stroma is similar in irises of all colors. The iris pigment epithelium forms the densely pigmented posterior layer of the iris. It consists of two layers of tightly fused, pigmented cells.

Differences in iris color depends on the amount of pigmentation in the deep stroma, especially the anterior border layer, and on the density of the stroma, both of which influence how much light, and what wavelengths, are absorbed and reflected. As with other objects, the color we see is the result of reflected light. The stroma of brown irises is densely pigmented with melanin and absorbs much of the light that enters it. In many human populations, brown is the only eye color. Blue irises have lightly pigmented stroma, and light of longer wavelengths (red to yellow) readily penetrates the iris and is absorbed, while some light of shorter wavelength (blue) is reflected back and scattered by the iris stroma; hence the blue color.

The inability to make melanin, as in albinism, leaves the iris without any pigment. The iris appears pink from the color of the blood flowing through it. Albinism is a recessive condition, requiring two defective alleles for melanin production, one inherited from each parent. Albinism also prevents pigment production in the hair and skin.

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