Pigments in the Human Body: Functions and Health Problems

Functions of Pigments in the Body

A pigment is a chemical that has a specific color. Biological pigments color our body and its products, but this isn't their primary function. The chemicals often play vital roles in the daily operation of the body. For example, melanin is a yellow-to-black pigment in our skin that helps to protect it from sun damage. Rhodopsin is a purple chemical in our eyes that enables us to see in dim light. Hemoglobin is a red pigment in blood that carries oxygen from our lungs to our cells.

Some pigments in our bodies are waste products and appear to have no function. Others are very important to our well-being and even to our survival. In some cases, health problems can develop if too much pigment collects in the body or if too little is made.

Melanin in the Skin

Melanin is the main pigment in the skin, where it’s made by cells called melanocytes. Two forms of skin melanin exist—eumelanin, which is brown or brown-black, and pheomelanin, whose color ranges from yellow to red. The molecules of melanin are present in various proportions in the skin of different people to produce the range of human skin colors. Blood vessels in the skin also contribute to skin color due to the presence of hemoglobin, a red pigment in blood.

Melanin is deposited near the surface of the skin. It absorbs dangerous ultraviolet rays from the sun, preventing the UV light from traveling deeper into the skin. Ultraviolet light can cause DNA damage in cells as well as skin cancer, so melanin is an extremely important molecule. As noted below, however, it doesn't absorb all of the dangerous radiation that strikes our bodies. We all need to take precautions to prevent skin damage from sunlight, whatever the color of our skin.

Everyone has the same number of melanocytes, but some people make more melanin than others. If those cells make just a little bit of melanin, your hair, skin and the iris of your eyes can be very light. If your cells make more, then your hair, skin, and eyes will be darker.

— WebMD

Melanin Concentration

When light-colored skin is exposed to intense sunlight, it responds by making more melanin than usual. The extra melanin provides additional (but not complete) protection from UV damage and gives the skin a tanned appearance. Although a tan is often considered to be desirable, it's an indication that the skin has been under stress from sunlight exposure.

Since dark-colored skin already contains a lot of melanin before being exposed to sunlight, it provides more protection from sun damage than light-colored skin. However, this protection still isn't complete. Dermatologists say that people of all skin colors should wear sunscreen.

Melanin in the Hair and the Iris of the Eye

Hair Color

Melanin is found in other areas of the body besides the skin. Both eumelanin and pheomelanin contribute to the color of hair. The eumelanin exists in two varieties—brown eumelanin and black eumelanin. Pheomelanin colors the hair yellow or orange. The proportions of these pigments determine hair color.

Structure of the Iris

Melanin also plays a role in determining the color of the eye. The iris is the coloured part of the eye. The outer and thicker layer of the iris is called the stroma. Behind this is a thin layer called the iris pigment epithelium. The pigment epithelium contains melanin. The stroma may or may not contain the chemical.

The stroma of the iris plays an important role in determining our eye color. It contains collagen fibers, melanocytes in some cases, and other cells in a loose arrangement. Blue-eyed people have no melanocytes in their stroma.

Eye Color

Iris color is determined by a combination of factors related to the stroma, including the density and arrangement of the collagen fibers and stroma cells, the number of melanocytes and the amount of eumelanin in them, and the ability of the stroma to scatter light with a long wavelength, which appears blue in color to us.

People with brown eyes generally have the highest concentration of melanin in their stroma. People with green eyes have an intermediate amount. The smaller amount of melanin combined with the ability of the stroma to scatter light produces a green color. The scattering of light plays a major role in creating the color of blue-eyed people.

Melanin is also present in the inner ear, where its function is unknown. A dark form of melanin called neuromelanin is found in parts of the brain.

Recommended

Emily Dickinson's "There is a morn by men unseen"

Rhodopsin in the Rods of the Retina

Activity of Rhodopsin

Several pigments are present in the eye and are essential to its function. Rhodopsin is located in the rod cells of the retina. The retina is the light-sensitive layer at the back of the eyeball. Rhodopsin is also known as visual purple due to its color. It functions in dim light and enables us to see shades of grey. In bright light, rhodopsin is bleached and breaks up into retinal and a protein called opsin. In the dark, the process is reversed, and rhodopsin is regenerated.

Importance of Beta-Carotene

Since retinal is made from vitamin A, this vitamin is an essential nutrient for night vision. Beta-carotene is a yellow or orange plant pigment, which our bodies can convert into vitamin A. The chemical is especially abundant in carrots, so the old myth that carrots are good for night vision is actually true. Pumpkin purée and orange sweet potatoes (yams) are also great sources of beta-carotene. Green leafy vegetables often are as well. Here, the orange pigment is hidden by the chlorophyll in the leaves.

Beta-Carotene Precautions

It’s not safe to eat large amounts of pre-formed vitamin A, which is toxic at high levels, but eating a large but not excessive amount of beta-carotene doesn’t seem to be dangerous. It's important to keep in mind that eating an excessive amount of even a normally beneficial nutrient could be harmful.

Research suggests that while smokers can eat foods containing beta-carotene, they shouldn’t ingest supplements, which may increase the risk of lung cancer. The same is true for people who have had long-term exposure to asbestos fibers.

Carotenemia

Eating large amounts of beta-carotene in food or supplements can result in some of the pigment being deposited in the skin, giving it a yellow color. This condition is called carotenemia and is thought to be harmless, at least in the short term. It is an indication that the pigment level in the diet should be reduced.

The cause of yellow skin should be investigated by a doctor, especially if it remains after dietary changes. The color change may be due to a health problem instead of diet, even if the affected person does eat a lot of beta-carotene.

Cone Pigments in the Retina of the Eye

The cone cells in the retina respond to bright light and enable us to see color and detail. Humans have three types of cone cells, which are known as the S, M, and L cones. Each type responds best to a specific range of light wavelengths, although there is some overlap in cone sensitivity.

• S cones are most sensitive to the shorter wavelengths of light, which produce a blue color, and are sometimes called blue cones. This alternate name is a bit confusing because S cones respond to blue light but are not blue in color.
• M cones, or green cones, are more sensitive to medium wavelengths, which produce green light.
• The L cones, or red cones, respond best to long wavelengths, which produce red light.

The cone pigment molecules are called iodopsins and are chemically similar to rhodopsin. Vitamin A is required for the manufacture of the iodopsins, so this vitamin is important for color vision as well as for night vision. Each of the three types of cones contains its own version of iodopsin.

Zeaxanthin and Lutein in the Eye

The central part of the retina provides very detailed vision and is known as the macula. When we look directly at something, the reflected light rays from the object strike the macula. The central portion of the macula has the best vision in the retina and is called the fovea centralis (or sometimes just the fovea). The fovea contains cones but no rods. This is why when we're outdoors at night, it's useful to look at objects from the side of our visual field rather than looking directly at the objects. This allows reflected light rays from the objects to fall on the outer portion of the retina, which has rods.

Zeaxanthin and lutein are yellow pigments in the macula. These two pigments belong to the carotenoid family, just as beta-carotene does, and give the macula a yellow appearance. They are thought to help maintain the health of the macula by protecting it from light damage and possibly by reducing oxidative stress. It's known that when people ingest zeaxanthin and lutein, the levels of these pigments in the macula increase. Eggs are a good source of zeaxanthin and lutein, and so are corn and green leafy vegetables.

Age-Related Macular Degeneration (AMD or ARMD)

Age-related macular degeneration is the leading cause of vision loss in older people. As their macula degenerates, it becomes harder for a person to see a clear image. In people with AMD, the macula has a lower level of zeaxanthin and lutein than in people without AMD. Scientists suspect—but don't know for certain—that ingesting more zeaxanthin and lutein will decrease the chance of AMD development and may help to prevent the disorder from getting worse once it has started.

Hemoglobin

Hemoglobin is a red protein and pigment inside red blood cells that transports oxygen around the body. The hemoglobin is responsible for the blood’s color. One hemoglobin molecule joins to four oxygen molecules.

A normal red blood cell contains 250 million to 300 million hemoglobin molecules. Since there are 4 million to 6 million red blood cells per microliter of blood in a healthy person (one microliter = one-millionth of a liter), a lot of oxygen travels through the blood. This oxygen is an essential nutrient for the estimated 50 to 100 trillion cells in the human body. These cells need oxygen to produce energy from digested food.

Bile Pigments

Red blood cells live for about 120 days and are then broken down by the liver and spleen. Their hemoglobin is changed into a green pigment called biliverdin. Biliverdin is then changed into yet another pigment known as bilirubin, which is yellow. Bilirubin enters a liquid called bile, which is made in the liver.