Tuesday, April 7, 2009
Skin Color
On page 171, Carroll discusses melanin and its affect on the skin color. How does melanin interact with other hormones (feedback mechanisms)and what is its job in humans? Also, what effect does melanin have on mutations in humans? If UV radiation is so bad, why would natural selection promote our melanin pigments? Also, how does melanin affect vitamin D production in the body?
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ReplyDeleteMelanin as discussed in the book is pigment that is released by cells in the skin called melanocytes. Melanin is generally used like a natural sunblock, which helps prevent the skin from burning and of UV damage with too much sun exposure.
ReplyDeleteIn animals such as amphibians and fishes, melanocytes-stimulating hormone (MSH) is released by the intermediate lobe of the pituitary gland. This hormone stimulates the response of melanin to be motored by kinesin out to the microtubules of the melanocytes. This spreads the pigments to cover the skin and create a darker appearance. When hormone release is stopped the melanin will then gradually be motored back from the microtubules to the center of the melanocytes by dynein.
Normally melanocytes are found in the stratum basale of the epidermis. When higher concentrations of UV light are placed on the skin or body the hormone MSH is released by the pituitary to create the melanin pigments that darken the skin. This works because the MSH binds to melanocortin-1 receptors (MC1R) that once bound to, stimulate the process of melanin. A reason for a tan or the skin to become darker from exposure to UV light is because UV light is an inducer to the synthesis of MC1R and MSH. If both of these levels increase the amount of melanin in the skin would also increase causing the skin to have a darker tone.
Melanin is important to the body because of its ability to work as this natural sun block. The melanin will absorb the UV rays of the sun that are normally very harmful to the body and spread it as harmless heat rather than the harmful UV rays. UV rays can be harmful to the DNA inside the cell indirectly through such things as oxygen radicals or hydroxyls from UVA, which does not burn the skin and so cannot be tested using SPF tests. From UVB there can be direct interference with DNA as the UVB rays excite the DNA and cause covalent bonds to form between adjacent cytosine bases. This causes the cell, during DNA replication, to read a code that normally was “CC” as “AA” and this leads to an added “TT” to the DNA. This is a mutation that occurs in the DNA of the skin cells that are affected by the UVB rays and can cause cancerous growth to occur. This can be prevented to an extent by the use of melanin. This is why the production of melanin is a selective advantage to humans. By absorbing UV rays and turning them into heat, melanin is able to prevent damage to the DNA of skin cells to a certain degree. With no melanin there would be a sharp incline in the rates of skin cancer and the amount of mutations that occurred in skin cells.
Because of these benefits of melanin there have been mutations that have occurred in humans. This can be seen today through the different colors of different ethnicities around the globe. In early civilization humans developed mutations and through natural selection these mutations became dominant. Just as different animals that inhabit different countries and areas of the world have adapted to live in areas with a higher water resource or lower water resource or high abundance in other sources and low abundance in others, humans have adapted to the amount of UV light that they are exposed to at a given region. Those areas that were closer to the equator and thus were exposed to a higher concentration of UV light needed full protection from the harm and damage that may have arisen. The mutations leading to different amount of genes that properly express the MC1R proteins can be explained in different ways. The first being that countries closer to the equator with more sun exposure used all the genes in protecting their skin and because of this the genes were less prone to mutations. On the other hand those who lived with less sunlight needed less protection and so some genes were unused and so had a higher affinity to mutation. This is why the ratio of intact MC1R genes to mutated genes are varying, the ratio being highest in darker skinned people that needed melanin most. Another explanation may be that since melanin is important for vitamin D production, those humans that were not greatly exposed to the sun needed all the UV light they could get in order to produce vitamin D. This caused them to not use the genes as much because they wanted more UV light. Both ways show the sense of the mutation of the genes in humans and how natural selection allowed for them to adapt to their specific environment.
Speaking of vitamin D production, melanin plays a key role in the production of vitamin, which helps in keeping the skeletal system of the body strong, and also works in the growth of the bones. This is because vitamin D levels are correlated with calcium absorption of the body. With no vitamin D the body is unable to properly absorb calcium and so the bones are left weak and unable to fully grow. Vitamin D deficiency is a cause for a range of cancers. UVB rays work in the cells by triggering the change of a cholesterol-related molecule. This molecule, located in membranes of skin cells, releases vitamin D, which then travels through the circulatory system to the kidney where it is changed into 25-hydroxyvitamin D (calcitrol). This hormone, calcitrol, works with parathyroid hormone to regulate calcium concentrations in the blood. The calcitrol makes sure that calcium absorption is taking place in the intestine and also when necessary goes to the bones for calcium release when levels are low in the blood.
below is link to a flow chart showing uses of vitamin D in body
http://www.healingtherapies.info/VitDmetabolism.jpg
http://www.medterms.com/script/main/art.asp?articlekey=4340
http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/M/MSH.html
http://en.wikipedia.org/wiki/Melanin
http://en.wikipedia.org/wiki/Ultraviolet#Evolutionary_significance
http://www.healingtherapies.info/Sunlight&VitaminD.htm
Melanin which is found in plant, animal, and protista kingdoms, where it serves predominantly as a pigment. The class of pigments are derivatives of the amino acid tyrosine. Many melanins are insoluble salts and show affinity to water. The most common form of biological melanin is eumelanin, a brown-black polymer, dihydroxyindole carboxylic acid. Another common form of melanin is pheomelanin, a red-brown polymer of benzothiazine units largely responsible for red hair and freckles. The increased production of melanin in human skin is called melanogenesis. It is stimulated by the DNA damages that are caused by UVB-radiation, and it leads to a delayed development of a tan. This melanogenesis-based tan takes more time to develop, but it is long lasting. The photochemical properties of melanin make it an excellent photoprotectant. It absorbs harmful UV-radiation and transforms the energy into harmless amounts of heat through a process called "ultrafast internal conversion". This property enables melanin to dissipate more than 99.9% of the absorbed UV radiation as heat. This prevents the indirect DNA damage which is responsible for the formation of malignant melanoma also known as skin cancer.
ReplyDeleteIn humans, melanin is the primary determinant of human skin color and also found in hair, the pigmented tissue underlying the iris, the medulla and zona reticularis of the adrenal gland, the stria vascularis of the inner ear, and in pigment-bearing neurons within areas of the brain stem, such as the locus ceruleus and the substantia nigra.
Dermal (skin) melanin is produced by melanocytes, which are found in the stratum basale of the epidermis. Although human beings generally possess a similar concentration of melanocytes in their skin, the melanocytes in some individuals and ethnic groups more frequently or less frequently express the melanin-producing genes, thereby conferring a greater or lesser concentration of skin melanin. Some individual animals and humans have very little or no melanin in their bodies, a condition known as albinism. Because melanin is an aggregate of smaller component molecules, there are a number of different types of melanin with differing proportions and bonding patterns of these component molecules. Both pheomelanin and eumelanin are found in human skin and hair, but eumelanin is the most abundant melanin in humans, as well as the form most likely to be deficient in albinism. Melanocytes insert granules of melanin into specialized cellular vesicles called melanosomes. These are then transferred into the other skin cells of the human epidermis. The melanosomes in each recipient cell accumulate atop the cell nucleus, where they protect the nuclear DNA from mutations caused by the ionizing radiation of the sun's ultraviolet rays. People whose ancestors lived for long periods in the regions of the globe near the equator generally have larger quantities of eumelanin in their skins. This makes their skins brown or black and protects them against high levels of exposure to the sun, which more frequently results in melanomas in lighter skinned people.
With humans, exposure to sunlight stimulates the skin to produce vitamin D. Because high levels of cutaneous melanin act as a natural sun screen, dark skin can be a risk factor for vitamin D deficiency in regions of the Earth known as cool temperate zones; i.e. above 36 degrees latitude in the Northern hemisphere and below 36 degrees in the Southern hemisphere. As a result of this, health authorities in Canada and the USA have issued recommendations for people with darker complexions including people of southern European descent to consume between 1000-2000 IU of vitamin D daily.
http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/M/MSH.html
http://www.wisegeek.com/what-is-melanin.htm
Melanin serves predominantly as a pigment and is the primary determinant of human skin color and hair color. The melanocyte stimulating hormone is produced in the anterior pituitary gland and it regulates activity of melanocytes such as skin cells with black pigment, or melanin. In humans it causes moles, freckles, and suntan or darkening of the skin. It is made from a pro-opiomelanocortin or a large protein cleaved into short fragments. However, when MSH is secreted by the mammalian pituitary gland then it works in fat metabolism and if not enough MSH is present then obesity can occur. Melanin is also the pigment tissue underlying the iris, the medulla and zona reticularis of the adrenal gland, the stria vascularis of the inner ear, and in pigment-bearing neurons within areas of the brain stem. The melanocytes that produce melanin are found in the stratum basale of the epidermis. Some ethnic groups express the melanocytes in their skin more frequently than others therefore creating a greater or lesser concentration of skin melanin. Melanin is formed from smaller component molecules so there are different bonding patterns of these smaller molecules.
ReplyDeleteMelanin comes in two types, pheomelanin (red) and eumelanin (dark brown). Both of these types are determined by about four to six genes which operate with incomplete dominance. The evolution of the different skin tones came from the haired primate ancestors of humans like apes. Hominids then evolved harilessness and had dark skin which was required to prevent low folate levels because they lived in Africa, a country that gets a lot of sun. However, Inuit and Yupik have dark skin even though they live in a sun poor environment, and this occurred becuase of their fish-based diet that supplies a lot of vitamin D to their skin.
In addition, melanin is associated with the regulation of energy balance and body weight. An experiment was done in rodents that examined if mutations in the two known melanin-concentration hormone receptors may be connected to obesity in humans. There were 106 subjects with early onset obesity with two missense variants in the melanin concentration hormone receptor. They were missing Y181H and R248Q. It was shown that R248Q factors cosegregated with obesity across two geneartions and no evidence of ligand hypersensitivity for extracellular signal-regulated kinase phosphorylation was found. Therefore, mutations in MCH receptors are not found in humans with early onset obesity, so this mutation in humans did not change the role the melanin has on the body.
Natural selection promotes our melanin pigments because it would have been critical for the survival of primates in the heat and sun of the topical environment. They required melanin to survive and to protect their skin. Today however, there are ways to get around the use of melanin such as applying sunscreen or not being out in the sun all day, but evolution is slow to act so some people still produce more melanin then other people and they therefore have much darker skin.
Vitamin D is make in the skin photochemically, but the presence and concentration of melanin is critical. Melanin filters light going into the skin and the concentration of melanin is based of the UV light to reach the epidermal layer of the skin. Normally there is enough 7-dehydrocholesterol in the stratum spinosum and stratum basale of the skin to meet the body's production of vitamin D requirements. Melanin concentration does not change the amount of vitamin D that can be produced.
http://ods.od.nih.gov/factsheets/vitamind.asp
http://www3.interscience.wiley.com/journal/119938242/abstract
http://www.nature.com/oby/journal/v12/n5/full/oby200489a.html
http://en.wikipedia.org/wiki/Human_skin_color
ttp://www.google.com/search?client=safari&rls=en-us&q=melanin&ie=UTF-8&oe=UTF-8
When skin, especially lighter skin, is exposed to large amounts of sunlight, the pituitary gland produces melanocortin, a stimulant hormone that causes skin cells to produce melanin, a pigment that results in the darkening or skin cells. Higher concentrations of melanin make skin darker and protect it from harmful UV radiation, which damages cells, causing mutations in the base codes of our DNA. This is why lighter skinned people tan in the summer. The increased exposure to UV radiation triggers the release of melanocortin. Since UV radiation is so harmful to cells, causing cancer, wrinkles, pre-mature aging, dark spots, and other abnormalities, natural selection has resulted in people from areas of high UV radiation, such as Africa and parts of Asia and South America to have darker pigmented skin. This protects them UV damage because melanin absorbs UV radiation. As a photoprotectant, it converts the energy from this absorbed radiation and converts it into harmless heat energy through “ultrafast internal conversion”. The melanin, produced in melanocytes and transferred to the skin cells via melanosome, accumulates on top the cell nucleus, protecting the DNA inside from the indirect mutations caused by UV light.
ReplyDeleteNot all UV radiation is bad, however. While UV-B radiation causes mutations in our genes, UV-A radiation is necessary for our bodies to synthesize vitamin D. Vitamin D is necessary for the absorption of calcium and bone growth, and a deficiency in vitamin D can lead to several types of cancer as well as osteoporosis. Melanin blocks this beneficial radiation along with the harmful rays, and so dark-skinned people living in areas that do not get a lot of sunlight have a greatly increased risk of vitamin D deficiency. To combat this, vitamin D is added to our milk supply.
Lighter skinned people have lower concentrations of melanin in their cells because of the lower concentrations of UV radiation. In order for the skin to be able to synthesize enough vitamin D, lighter skin tones were selected for as having an advantage in vitamin D production. Darker people have more melanin in their cells because of the exposure to higher concentrations of UV radiation. Their DNA needed protection from the UV light and so darker skin tones were selected for. Because the UV light was so intense, the skin cells were still able to synthesize enough vitamin D. The complete absence of melanin is called albinism and is caused by a mutation in the MC1R gene, which codes for the production of melanin.
http://en.wikipedia.org/wiki/Human_skin_color
http://en.wikipedia.org/wiki/Melanin