Pigments in the Human Body: Functions and Health Effects
The 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 pigments 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 pigment in our eyes that enables us to see in dim light. Hemoglobin is a red pigment 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 wellbeing 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.
A Melanocyte in the Skin
Melanin in the Skin
Melanin is the main pigment in 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. These molecules 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 body. We still need to take precautions in sunlight.
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
Melanin is found in other areas of the body besides the skin. Both eumelanin and pheomelanin contribute to the color of hair. Eumelanin exists in two varieties—brown eumelanin and black eumelanin. Pheomelanin colors the hair yellow or orange. The proportions of these pigments determine actual hair color.
Structure of the Iris
Melanin also plays a role in determining the color 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 plays an important role in determining our eye color. It contains collagen fibers, melanocytes, and other cells in a loose arrangement. Blue-eyed people have no melanocytes in their stroma, however.
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.
Rhodopsin in the Rods of the Retina
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, rhodospin is bleached and breaks up into retinal and a protein called opsin. In darkness the process is reversed and rhodopsin is regenerated.
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. This pigment 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.
It’s not safe to eat large amounts of pre-formed vitamin A, which is toxic at high levels, but eating a large amount of beta-carotene doesn’t seem to be dangerous. Research suggests that while smokers can eat foods containing the nutrient, they shouldn’t ingest beta-carotene supplements, which may increase the risk of lung cancer. The same is true for people who have had a long-term exposure to asbestos fibers.
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 harmless. The cause of yellow skin should always be investigated, however, since the color change may be due to a health problem.
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 of cone 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 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.
Parts of the Eye
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 increases. 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 macular 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 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.
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.
The liver sends bile to the gall bladder. The gall bladder stores the bile and releases it into the small intestine (or small bowel) when fat is present in the intestine. Bile contains salts whose function is to emulsify ingested fats. This emulsification prepares the fats for digestion by enzymes.
Bile and food that is not digested pass from the small intestine into the large intestine. Here bacteria and chemical reactions change the bilirubin into a brown pigment called stercobilin. Stercobilin leaves the body in the feces. The pigment gives feces its color.
Some bilirubin is converted into urobilin, a yellow pigment that is absorbed through the intestinal lining into the bloodstream. The kidneys excrete the urobilin in urine, giving the liquid its typical yellow color.
There are many disorders that are caused by an insufficient or excessive amount of a pigment. Three of these disorders are vitiligo, jaundice, and iron-deficiency anemia. In vitiligo, melanin is lost from the skin. In jaundice, bilirubin collects in the skin. In iron-deficiency anemia, the blood lacks hemoglobin or the red blood cells that contain the hemoglobin.
Melanin Loss and Vitiligo
Vitiligo is a condition in which melanocytes in the skin are destroyed, resulting in white patches that contain no melanin. The cause of vitiligo is unknown, but it may develop due to the inheritance of specific genes that make a person susceptible to melanin loss. The most popular theory at the moment is that vitiligo is an autoimmune disease, however. In an autoimmune disease, the immune system mistakenly attacks the body's own cells—in this case, the melanocytes.
Bilirubin and Jaundice
Hyperbilirubinemia is a condition in which bilirubin becomes too concentrated in the body. As a result, bilirubin collects in the skin and the sclera (the white part of the eye), producing a yellow color known as jaundice.
Hyperbilirubinemia may develop if too many red blood cells are destroyed. This results in the breakdown of too much hemoglobin and the production of too much bilirubin. The disorder may also develop due to liver damage that prevents release of bilirubin into the small intestine or due to an obstruction in the passageways that transport bile.
Carotenemia and jaundice both involve yellowing of the skin. In carotenemia the sclera of the eye remains white while in jaundice it develops a yellow color.
Hemoglobin and Iron-Deficiency Anemia
Red blood cell and hemoglobin destruction, an insufficient amount of hemoglobin in the red blood cells, or the production of abnormal hemoglobin can cause a number of disorders, including several types of anemia. The anemia may be mild or severe.
The most common type of anemia is called iron-deficiency anemia. Hemoglobin contains iron and can't be made without this element. If the body lacks hemoglobin, an insufficient number of red blood cells will be produced and an inadequate amount of oxygen will be delivered to the body’s tissues. Iron-deficiency anemia can arise due to a diet that is low in iron, inadequate absorption of iron, or blood loss.
The main symptom of iron-deficiency anemia is fatigue, but other symptoms may be present as well. These include the craving to eat non-food substances, such as soil or ice. This condition is known as pica.
The Importance of Pigments in the Body
Melanin, zeaxanthin, lutein, hemoglobin, and the other pigments in our body are important molecules. Investigating their functions, mechanisms of action, and interactions with other components of the body is a very worthwhile activity. Discoveries made by scientists may lead to better treatments for health problems involving pigments. They may also give us a better understanding of how the body works.
Melanin and melanocytes description from the University of Bristol Palaeobiology Research Group
Your blue eyes aren't really blue from the American Academy of Ophthalmology
Information about rhodopsin and the eye from the School of Chemistry at the University of Bristol
Cones of the eye information from the NIH (National Institutes of Health)
Information about lutein and zeaxanthin from the American Optometric Association
Vitiligo facts from the Mayo Clinic
Information about age-related macular degeneration from the National Eye Institute
Jaundice facts from the Merck Manual Consumer Edition
Description of iron-deficiency anemia from the Mayo Clinic
Questions & Answers
Why does my daughter have brown eyes while the whites of her eyes are blue?
There are quite a lot of reasons why the sclera (the white part of the eye) turns blue. Sometimes it’s due to a thinner than normal sclera. Certain medications and diseases can cause the sclera to thin or to develop a blue color. This is why it’s important to visit a doctor to discover the reason for the color. It shouldn’t simply be accepted as normal or unimportant.Helpful 1
What is iodopsin?
The rods in our retina contain only a single visual pigment—rhodopsin. In contrast, cones include various pigments which respond to different wavelengths of light. The terms cone opsins, photopsins, or iodopsin are sometimes used as the general name for the cone pigments. The word iodopsin has a variable meaning, however. Different sources use it to mean different things concerning cone pigments.
© 2011 Linda Crampton