Pigmentation
The color of skin is influenced by a number of pigments, including melanin, carotene, and hemoglobin. Recall that melanin is produced by cells called melanocytes, which are found scattered throughout the stratum basale of the epidermis. The melanin is transferred into the keratinocytes via a cellular vesicle called a melanosome (Figure (PageIndex{3})) and (Figure (PageIndex{9})).
Melanin occurs in two primary forms: eumelanin exists as black and brown whereas pheomelanin provides a red color. Multiple genes are involved in determining how much and which type of melanin are produced, with dark-skinned individuals produce more melanin than those with pale skin. Exposure to the UV rays of the sun or a tanning salon causes melanin to be manufactured and built up in keratinocytes, as sun exposure stimulates keratinocytes to secrete chemicals that stimulate melanocytes. The accumulation of melanin in keratinocytes results in the darkening of the skin, or a tan. This increased melanin accumulation is protection from UV radiation, which could damage the DNA of epidermal cells and breakdown folic acid, a nutrient necessary for our health and well-being, circulating through the bloodstream in the dermis. In contrast, too much melanin can interfere with the production of vitamin D, an important nutrient involved in calcium absorption, since UV radiation is required for its production. Thus, the amount of melanin present in our skin is dependent on a balance between available sunlight and folic acid destruction, and protection from UV radiation and vitamin D production.
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It requires about 10 days after initial sun exposure for melanin synthesis to peak, which is why pale-skinned individuals tend to suffer sunburns of the epidermis initially. Dark-skinned individuals can also get sunburns, but are more protected than are pale-skinned individuals. Melanosomes are temporary structures that are eventually destroyed by fusion with lysosomes; this fact, along with melanin-filled keratinocytes in the stratum corneum sloughing off, makes tanning impermanent.
Too much sun exposure can eventually lead to wrinkling due to the destruction of the cellular structure of the skin, and in severe cases, can cause sufficient DNA damage to result in skin cancer. When there is an irregular accumulation of melanocytes in the skin, freckles appear. Moles are larger masses of melanocytes, and although most are benign, they should be monitored for changes that might indicate the presence of cancer (Figure (PageIndex{11})).
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Carotene, an orange/red pigment found in many vegetables such as carrots, can also have an affect on skin pigmentation. Once consumed, carotene is converted by the human body into vitamin A, which in turn is used for vision, immune system function, and to stimulate fibroblasts in the skin. Recall that fibroblasts produce the elastic and collagen fibers of the dermis, so having active fibroblast helps with the firmness of skin. However, the over intake of carotene can cause the skin to turn yellowish-orange in color, as a result of the excess being stored just under the skin.
Another factor that can affect the appearance of skin is the activity of hemoglobin in the blood stream. Hemoglobin is a pigment found in red blood cells that is responsible for carrying oxygen throughout the body. A sudden drop in oxygenation can affect skin color, causing the skin to initially turn ashen (white). With a prolonged reduction in oxygen levels, dark red deoxyhemoglobin (hemoglobin not carrying oxygen) becomes dominant in the blood, making the skin appear blue, a condition referred to as cyanosis (kyanos is the Greek word for “blue”). This happens when the oxygen supply is restricted, as when someone is experiencing difficulty in breathing because of asthma or a heart attack. However, in these cases the effect on skin color has nothing do with the skin’s pigmentation.
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