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Recession of skin tan

Recession of skin tan


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In early October I was on a holiday near the equator and got tanned. Now, roughly 3 months later, the tan on my face and arms has gone almost entirely where my belly, chest and legs are still moderately tanned. It seems that body parts that are exposed (to light) more than others seem to loose the tanning effect more quickly. Why is this? Or is there another reason?

I am not a biologist so I would appreciate an answer that is articulated in a way non-biologists can understand it.


Why Does the Body Tan?

Not to bronze your statuesque physique, although that may be why you tan. Tans are natural shields against the sun's ultraviolet radiation, which can damage skin tissue in the form of a sunburn (as well as cause cancer in the long-run).

Exposure to ultraviolet rays causes certain skin cells to produce the pigment melanin, which darkens through oxidation. Enough beach bumming and those cells will migrate closer to the skin's surface and produce more melanin, further darkening the skin into a suntan. It's no wonder our bodies have developed the ability to produce melanin.

The pigment absorbs ultraviolet radiation and defends against further penetration of skin tissue. In other animals melanin proves diversely useful. It absorbs heat, an essential for cold-blooded organisms. It colors bird feathers, fish scales and squid ink, and helps to conceal nocturnal animals. Melanin even absorbs scattered light inside the eye to sharpen vision.

But it appears that only humans will risk their skins for a little extra surface pigment.


College of the Liberal Arts

Department of Anthropology

WATCH
The Evolution of Skin Color

“My ultimate goal in this research is for people to understand that their skin color is a result of evolution. That’s it. Skin color has no connection to the evolution of other traits.”

Artist’s rendition of a map created by Nina Jablonski and George Chaplin showing predicted skin colors of human natives of various regions based on levels of ultraviolet radiation from the sun in each region. Cut-Paper Illustration by Gail McCormick


How Sunburns and Sun Tans Work

There is something mysterious about the sun and skin. Why is it that if you go out on a bright summer day and spend an hour in the sun, you get a sunburn? You get a sunburn, that is, unless you happen to have taken the time to get a gradual tan. With a tan you can go out in the sun and nothing happens. Of course, that doesn't apply if you have "fair skin." The fair skinned among us never get a tan, so they always get sunburned. Unless of course they are wearing a sunscreen.

Does this make sense? What exactly is a sunscreen? And what is a tan? What is the difference between a tan and a burn? Why can you spread a little blob of lotion on yourself and be protected, but if you forget you are miserable?

If you take the time to look at skin and sunlight in some detail, all of this actually does begin to make sense. You can learn a huge amount about your body in the process. So, that's what we'll do in this article. First, let's take a look at how skin works.

Skin is one of the most amazing organs in the human body. It is hard for us to think about it as an organ, however. We tend to think of organs as boxy things. Your heart, liver, kidneys - those are obviously organs. But skin is an organ too, especially if you look at the dictionary definition of "organ", like this definition from the Merriam Webster Collegiate Dictionary:

    Organ - a) differentiated structure (as a heart, kidney, leaf, or stem) consisting of cells and tissues and performing some specific function in an organism b) bodily parts performing a function or cooperating in an activity

By that definition, skin is definitely an organ. Skin is made up of very specific cells and tissues, and their collective purpose is to act as the boundary between "you" and "the world". One of the neat things about skin that makes it different from a lot of other organs is the fact that it does have to deal with the real world. Therefore it is loaded with sensors, and it also has a very tough layered design so that it can handle realities of the environment like abrasion and sunlight.

If you take a look at a cross section of typical skin (like the skin on your arm or leg) you find that it is made up of two main layers: the epidermis on the outside and the dermis on the inside. The epidermis is the barrier, while the dermis is the layer containing all the "equipment" -- things like nerve endings, sweat glands, hair follicles and so on. To the right is a picture to help you see what is going on.

In the subcutaneous layer (you may have heard of subcutaneous fat -- this is where it lives) you can see the blood vessels (shown as two thin red and blue lines). These vessels branch infinitely (not shown) into the dermis to supply the sweat glands, hair follicles, sebaceous glands and erector muscles with blood. They also fan out into the dermis's capillary bed. It turns out that the dermis is loaded with capillaries. Capillaries satisfy the nutritional needs of the cells in the dermis, and they also help the skin perform an important cooling function in humans. The epidermis has no direct blood supply, but instead is supported and fed by the dermis.

Learn more about the dermis on the next page and how it relates to melanoma and sun exposure.

The dermis is where the action is functionally. The dermis contains sweat glands, hair follicles (each with its own tiny little muscle so that your "hair can stand on end"!), nerve endings and so on. There are several different types of nerve endings:

  • Heat sensitive
  • Cold sensitive
  • Pressure sensitive sensitive
  • Pain sensitive

All these different nerve endings let you sense the world. They also help you protect yourself from burns, punctures and the like by warning you when something is damaging your skin.

The epidermis is your interface to the world, and it is actually quite interesting. It has two main layers, the inner of which is living and the outer of which is dead. The dead skin cells of the outer layer are what we can actually see, and they are constantly flaking off and being replaced by new cells being pushed outward.

Learn about the layers of skin on the next page.

The living, inner layer of the skin is called the malpighian layer. The malpighian layer creates the dead cells that we can see. It is in direct contact with the dermis, which feeds and supports it. The malpighian layer is our focus of attention actually, because it is here that the sun affects the skin during tanning. The malpighian layer is itself layered like this:

  • In direct contact with the dermis is the basal layer. If you have ever heard of a basal cell carcinoma (cancer), this is where it starts.
  • Above the basal layer is the spinous layer.
  • Above the spinous layer is the granular layer.

Above the granular layer is the stratum corneum. The stratum corneum is the outer layer of dead cells -- the cells that we see as our skin. The cells in this layer are filled with a protein called keratin. Keratin is a very interesting protein because it is tough -- horns, hair, hoofs, fingernails and feathers all gain their strength from keratin. The same stuff that your fingernails are made of actually forms your visible skin (but in a much thinner and more flexible layer). That is what makes your skin so tough. In parts of the body that get a lot of wear, like the palms and the feet, the stratum corneum is thicker to handle the abrasion.

Living among the basal cells in the malpighian layer is another type of cell called a melanocyte. Melanocytes produce melanin, which is a pigment that is the source of tanning. The melanocytes are actually where a tan comes from. Here is what the Encyclopedia Britannica has to say about melanocytes:

    "The appearance of the skin is partly due to the reddish pigment in the blood of the superficial vessels. In the main, however, it is determined by melanin, a pigment manufactured by dendritic cells called melanocytes, found among the basal cells of the epidermis. Their numbers in any one region of the body, which range from about 1,000 to more than 2,000 per square millimetre, are roughly the same within and between races the blondest whites have as many as the darkest blacks. Colour differences are due solely to the amount of melanin produced and the nature of the pigment granules. When the skin becomes tanned on exposure to sunlight, the melanocytes do not increase in number, only in activity." ("Integumentary Systems, Pigmentation", Britannica CD. Version 97. Encyclopaedia Britannica, Inc., 1997.)

Not only do melanocytes produce a tan, they are also responsible for the form of cancer called melanoma. Melanoma is caused by UV radiation damage to melanocytes. Repeated exposure to UV can cause cancerous mutations.

The dermis is where the action is functionally. The dermis contains sweat glands, hair follicles (each with its own tiny little muscle so that your "hair can stand on end"!), nerve endings and so on. There are several different types of nerve endings:

  • Heat sensitive
  • Cold sensitive
  • Pressure sensitive sensitive
  • Pain sensitive

All these different nerve endings let you sense the world. They also help you protect yourself from burns, punctures and the like by warning you when something is damaging your skin.

The epidermis is your interface to the world, and it is actually quite interesting. It has two main layers, the inner of which is living and the outer of which is dead. The dead skin cells of the outer layer are what we can actually see, and they are constantly flaking off and being replaced by new cells being pushed outward.

The living, inner layer is called the malpighian layer. The malpighian layer creates the dead cells that we can see. It is in direct contact with the dermis, which feeds and supports it. The malpighian layer is our focus of attention actually, because it is here that the sun affects the skin during tanning. The malpighian layer is itself layered like this:

  • In direct contact with the dermis is the basal layer. If you have ever heard of a basal cell carcinoma (cancer), this is where it starts.
  • Above the basal layer is the spinous layer.
  • Above the spinous layer is the granular layer.

Above the granular layer is the stratum corneum. The stratum corneum is the outer layer of dead cells -- the cells that we see as our skin. The cells in this layer are filled with a protein called keratin. Keratin is a very interesting protein because it is tough -- horns, hair, hoofs, fingernails and feathers all gain their strength from keratin. The same stuff that your fingernails are made of actually forms your visible skin (but in a much thinner and more flexible layer). That is what makes your skin so tough. In parts of the body that get a lot of wear, like the palms and the feet, the stratum corneum is thicker to handle the abrasion.

Living among the basal cells in the malpighian layer is another type of cell called a melanocyte. Melanocytes produce melanin, which is a pigment that is the source of tanning. The melanocytes are actually where a tan comes from. Here is what the Encyclopedia Britannica has to say about melanocytes:

    "The appearance of the skin is partly due to the reddish pigment in the blood of the superficial vessels. In the main, however, it is determined by melanin, a pigment manufactured by dendritic cells called melanocytes, found among the basal cells of the epidermis. Their numbers in any one region of the body, which range from about 1,000 to more than 2,000 per square millimetre, are roughly the same within and between races the blondest whites have as many as the darkest blacks. Colour differences are due solely to the amount of melanin produced and the nature of the pigment granules. When the skin becomes tanned on exposure to sunlight, the melanocytes do not increase in number, only in activity." ("Integumentary Systems, Pigmentation", Britannica CD. Version 97. Encyclopaedia Britannica, Inc., 1997.)

Not only do melanocytes produce a tan, they are also responsible for the form of cancer called melanoma. Melanoma is caused by UV radiation damage to melanocytes. Repeated exposure to UV can cause cancerous mutations.

  • UVA (315 to 400 nm), also known as black light, which causes tanning
  • UVB (280 to 315 nm), which causes damage in the form of sunburn
  • UVC (100 to 280 nm), which is filtered out by the atmosphere and never reaches us.

One of the interesting things about UV radiation is that it is reflected by different surfaces. These reflections can amplify the effects of UV exposure. For example, snow reflects 90% of UV light. That is why you can get snow blindness and severe sunburns from skiing on a sunny day. Sand can reflect up to 20% of UVB that hits it, meaning that you can get extra UV exposure at the beach.

On the other hand, certain things absorb almost all UV radiation partially or completely. Glass is one of these substances -- many glasses are very good absorbers of UV (which is why you may have heard that you cannot get sunburn in a greenhouse -- just make sure it is glass and not plastic covering the greenhouse!). Most sunscreens use chemicals that have the same UV-absorbing properties.

So, now that we know all about the skin we can start to actually understand tans and sunburns. When you get a tan, what is actually happening is that the melanocytes are producing melanin pigment in reaction to ultraviolet light in sunlight. Ultraviolet light stimulates melanin production. The pigment has the effect of absorbing the UV radiation in sunlight, so it protects the cells from UV damage. Melanin production takes a fair amount of time -- that is why most people cannot get a tan in one day. You have to expose yourself to UV light for a short period of time to activate the melanocytes. They produce melanin over the course of hours. By repeating this process over 5 to 7 days pigment builds up in your cells to a level that is protective.

The previous paragraph applies to Caucasians. In a variety of other races, melanin production is continuous, so the skin is always pigmented to some degree. In these races the incidence of skin cancer is much lower because cells are constantly protected from UV radiation by melanin.

Melanocytes actually produce two different pigments: eumelanin (brown) and phaeomelanin (yellow and red). Red heads happen to produce more phaeomelanin and less eumelanin, which is why they don't tan very well. In albinos, the chemical pathway that produces melanin cannot proceed because an enzyme called Tyrosinase is missing. Therefore albinos have no melanin in their skin, hair or irises.

Melanocyte-stimulating hormone (MSH) is produced by the pituitary gland. MSH flows through the bloodstream and reaches the melanocytes, encouraging them to produce more melanin (for example, a person injected with a large dose of MSH will get darker). The pituitary gland is actually quite interesting -- it is tied into the optic nerve, which means that it can sense light. If you have ever raised chickens for eggs, you know that in the winter egg production falls way off. You solve this problem by providing light in the chicken coop. The extra light stimulates the pituitary gland in chickens, which causes the gland to produce a hormone essential to egg laying. In humans, light affects the pituitary gland as well and one result is the production of MSH. A funny side-effect of all of this is that wearing sunglasses may make you more susceptible to sunburn! See this page for some thoughts on the subject.


Can You Be Addicted to Tanning? New Study Says Yes

The concept of using machines to tan has been around since the early 1920s, when at-home tanning devices were introduced as a cure-all for various ailments. Fast forward nearly 60 years, when the first tanning salon opened in the U.S., starting a trend that would become widely popular over the next three decades. By 2009, there were 18,000 tanning salons in the U.S. In recent years, tanning’s popularity has begun to weaken, as the public has become more aware of the dangers surrounding it. Still, as of 2016, there were nearly 9,500 tanning salons serving approximately 30 million customers each year.

The medical community and organizations like The Skin Cancer Foundation have been warning people for years to stop tanning. Hundreds of former tanners who became skin cancer patients have shared their stories online and cautioned people not to make the same mistakes.

So why do some people continue to tan? New research confirms that for some, quitting tanning is not that simple.

According to a 2017 study conducted by researchers at Georgetown University Medical Center, 20 percent of women who tan show signs of tanning dependency. The researchers ran 400 women through a set of mental and behavioral tests both before and after a tanning session. Those who were labeled as having an addiction strongly believed in the physical and mood-boosting benefits they experienced after a tanning session. They were convinced it enhanced their appearance and attitude, ultimately making them feel better about themselves. However, once that feeling faded, the tanning bed users showed signs of depression.

“Understanding why people feel compelled to tan is important, because it helps physicians and other health care advocates develop better intervention techniques that encourage people to stop tanning,” says Deborah S. Sarnoff, MD, president of The Skin Cancer Foundation. “There is no such thing as a healthy UV tan. Whether you’re laying out on the beach or in a tanning bed, the damage your skin sustains can lead to skin aging and potentially deadly skin cancer.”

As with many addictions, this one has some seriously dangerous side effects, in addition to premature wrinkles, leathery skin and hyperpigmentation. Just one indoor tanning session before the age of 35 increases a person’s risk of melanoma, the deadliest form of skin cancer, by 75 percent. One study observing 63 women diagnosed with melanoma before age 30 found that 61 of them (a shocking 97 percent) had used tanning beds.

Dr. Sarnoff is passionate about this issue she says if she had it her way indoor tanning salons would be against the law. We asked her to provide a few tips for people who are having a hard time giving up tanning:


Sunburn and Suntan: What’s the Difference?

We all are very conscious about our skin. Most of us are unaware of the fact that skin is also an organ. It is considered to be the most interesting organ of the human body, as it can respond directly to the environment. But, at the same time, there are negative aspects of this response. Skin is the only organ in the human body that is exposed to the outer environment. Hence, it is more likely to get damaged by environmental factors, rather than any other internal organs of our body.

The most common damages that skin is likely to prone to are: sunburn and suntan. I know that most of you are thinking that why the term is being written twice, when it has the same meaning? This is the biggest misconception in the minds of the people that both sunburn and suntan are similar. But, these are two entirely different terms, with different consequences on the skin.

Today, I am going to tell you in detail the difference between the two, and how can you protect yourself from both the skin ailments?

Let’s Understand a Little Bit about Our Skin

As I have already told you, skin is an organ and it protects your inner body from the harmful agents, by acting as a barrier. Skin is loaded with various sensors that respond according to different situations, like sweat glands will produce sweat, only when, there is heat around you. One cannot sweat in a hill station at a temperature of 3 degrees Celsius, unless, one suffers an attack or something similar. Skin is not just single layered, but, is multi layered and each layer has its own specific role.

Broadly, there are 3 main types of skin layers- Epidermis (outer most layer), Dermis (layer next to epidermis), and subcutaneous tissue. The skin also has several sub layers.

How Does Sunlight Penetrate the Skin?

Sunlight arrives on the earth in three basic forms: infrared rays, visible light and UV (ultraviolet rays). Out of these three, UV rays are the leading cause of sunburns and suntans. Let’s look at the classification of UV rays:

  • UVA rays (315 to 400 nm) also called as ‘black light’ and causes sun tanning.
  • UVB rays (280 to 315 nm) that cause sun burn.
  • UVC rays (100 to 280 nm) that never reach us.

Most of the time, we get affected by UVA, which are present near sea levels, and geographic regions with more sun exposure. However, UVB rays cause major problems on our skin, like ageing, skin cancer, wrinkles and much more. Even UVA are said to cause all such things, but, UVB are considered to be more hazardous.

How Does a Person Get Suntan?

When you get exposed to sunlight, melanocytes (special skin cells) produce a pigment called as melanin, as we discussed in our previous article Freckles. UV radiations stimulate the production of melanin, which absorbs UV radiations. This melanin is responsible for turning your skin color brown that is called as ‘tan’. Some people tend to produce melanin quite often, while some take a few days to start producing melanin. That is the reason, why some get tanned in a day, while some take weeks to get tanned.

Melanocyte is stimulated by the pituitary gland that has Melanocyte Stimulating Hormone (MSH). For example, when you are exposed to sunlight, MSH will run through your blood, reach melanocytes and will prompt them to produce melanin. More the MSH stimulation in your body, darker will be your skin. Interestingly, pituitary gland is connected to optic nerves, which means whenever your eyes are exposed to sunlight pituitary automatically stimulates MSH, which will then flow through blood and accounts for melanin secretion from melanocytes.

This is the reason, why wearing sunglasses increase the risk of sun damage in a person’s body. I am going to tell you about this in my upcoming articles.

If Suntan is all about Developing Brown Color, then What is Sunburn?

When you are overly exposed to sunlight, your skin cells are likely to get killed. Body responds to this damage by circulating more blood via capillaries in the affected areas, which accounts for redness and sometimes, even blisters accompanied by the pain. This is the reason, when you press a sunburn area, it becomes pale, and when you release your hand, it becomes reddish again.

Both, “suntan” and “sunburn” are bad for skin, and both carry the risk of skin cancer. One thing that I want to make clear is that there is no such term called ‘healthy tanning’. So, now, I have cleared you the differences between sunburn and suntan. But, always protect yourself from both, as none of them is good for us.


THE PIGMENTATION PROCESS

MELANIN – the pigment

Melanin is a pigment that produces different colors in skin and hair. It is a pigment that protects you .

By conferring color to skin, melanin protects skin from damaging UV rays. It functions like an antioxidant by quenching free radicals. In fact, it is an efficient antioxidant .

There are 2 kinds of melanin:

  1. Eumelanin – yellow to brown to very dark brown (almost black) color seen in brown-black hair
  2. Pheomelanin – red-yellow color seen in red hair

The amount of each type of melanin partly determines your skin and hair color.

MELANOCYTES – produce the pigment

Melanin is produced by special cells called MELANOCYTES located in the basal layer of the epidermis.

Melanin production begins when there is a trigger, such as UV radiation . This is why you tan and get sun spots from being in the sun.

Melanocytes perform several important functions:

  • scatter UV light
  • absorb heat
  • neutralize Reactive Oxygen Species
  • protect DNA from UV damage
  • protect cell membranes from oxidation

These melanocytes have octopus-like tentacles (dendrites) that carry melanin to nearby skin cells (keratinocytes) when they are needed (i.e. when skin is exposed to the sun’s damaging UV rays).

The melanin that is deposited in the skin cells serves to protect the DNA .

If your skin tans , it is a sign that you already have DNA damage. This is why there is no such thing as safe tanning. By definition, a tan is a sign of skin damage.

When you exfoliate or get ablative procedures done (lasers), the pigment that comes off first is the pigment in the stratum corneum . Pigment can still re-appear later because it is produced in the layers below and takes a long time to migrate to the outermost layer of skin.

MELANOSOMES – carry the pigment

Melanin granules are contained in organelles called MELANOSOMES . The melanosomes are like parcels. These melanosome parcels move along the dendrites into nearby keratinocytes where they can continue to migrate upward through the epidermis.


Recession of skin tan - Biology

Evolutionary history is more than skin deep
March 2014

Many of the marks that evolutionary history has left on our bodies are invisible. Lactose tolerance, a predisposition towards diabetes, genes that contribute to breast cancer, and many other inconspicuous traits are legacies of the paths that our ancestors took as they left or stayed in Africa between 60 and 125 thousand years ago. However, other markers of these unique evolutionary histories are perfectly obvious, perhaps most notably skin color. It's clear that people whose ancestors hail from different parts of the earth have differently colored skin and that this is related to how much of the sun's radiation hits that part of the planet. The less radiation, the lighter the native population's skin color tends to be. This is a great example of recent evolution in human populations. But what if we go back deeper in our evolutionary history, back to when all of humanity lived in Africa? At that time, all humans had darkly pigmented skin. A new study sheds light on how and why this skin pigmentation evolved.

Where's the evolution?

Humans have different skin colors because we have different amounts and kinds of the pigment melanin in our skin. Our closest living relatives, the chimpanzees, have pale skin without melanin underneath their dark fur, and almost certainly the ancestor that we share with chimps did too. So how did the early members of the human branch of the tree of life get from hair-covered light skin to hairless dark skin? Researchers have many competing hypotheses about what sort of natural selection caused dark skin to evolve. In all of these hypotheses, the notion of evolutionary fitness is important.

In evolutionary terms, fitness indicates not how physically fit or healthy an organism is but how effective an organism carrying particular gene versions is at getting offspring into the next generation. So, for example, an animal carrying genes that cause it to expend little energy on reproduction and lots of energy on building muscles might look quite fit in the everyday meaning of the word, but have low evolutionary fitness because it produces far fewer offspring than other members of its species. If a particular gene version confers high fitness, that means that it helps organisms get offspring into the next generation&mdashand those offspring are likely to carry copies of the helpful gene version that their parent had. Hence, gene versions that confer high evolutionary fitness are likely to become common through the action of natural selection.

Scientists reason that sometime after our lineage separated from that of the chimpanzees, dark skin must have become common because it conferred a fitness advantage. But what exactly was that advantage? One prominent idea is based on the fact that exposure to UV radiation destroys folate&mdasha molecule that our bodies need for a wide variety of processes. For example, a lack of folate during pregnancy is known to contribute to birth defects like spina bifida. Darkly pigmented skin protects folate from being broken apart. Perhaps, as our ancestors lost their protective body hair, individuals without pigmentation genes suffered folate shortages that caused them to produce fewer and less healthy offspring. In that scenario, any person who happened to carry gene mutations that produced skin with protective pigmentation would have left behind more offspring and had a fitness advantage over those without skin pigmentation. Hence, over many generations, the genes that produce pigmented skin would have spread through our ancestral population.

It's difficult to figure out whether preventing folate destruction was the main reason that dark skin became common or whether it was some other benefit or a combination of different benefits. Now, research suggests that a factor previously written off may have helped select for pigmented skin after all: skin cancer. It's no secret that pigmentation protects us from skin cancer. African Americans are about 1/10th as likely to be diagnosed with a malignant melanoma and less than 1/50th as likely to be diagnosed with nonmelanoma skin cancer as are white Americans. However, this was not thought to be an important factor in the evolution of dark skin because the vast majority of these skin cancers strike when a person is well past reproductive age. This may be devastating to the health of the victim, but it has little impact on his or her evolutionary fitness. After all, evolutionary fitness is all about how many offspring one leaves behind in the next generation. If a disease strikes after a person has already reproduced to their full potential, it generally has little effect on evolutionary fitness. Because of that, few scientists thought that protection from skin cancer could have significantly contributed to the evolution of skin pigmentation in our ancestors. However, now that assumption is being brought into question.

The new study brought together many different lines of prior research focused on skin cancer rates among albino Africans living in central Africa&mdashthe same area where our ancestors first evolved pigmented skin. Most people with albinism carry mutations that cause them to produce no or very little melanin in their skin. As one would expect, these individuals run a much higher risk of developing skin cancer than do normally pigmented individuals. In fact, the risk is so high that many of them develop life threatening skin cancers before and during their reproductive years. In one study of more than 500 Tanzanians with albinism, nearly all died of skin cancer before the age of 40. Overall, the data suggest that more than 90% of albino individuals living near the equator in Africa will die in their 30s or before, mainly because of skin cancer. In other words, skin cancer does have the potential to cut short an individual's reproductive years&mdashand hence, could have an impact on evolutionary fitness.

What does all this indicate about our African ancestors? Some scientists reason that there are a lot of similarities between our pale-skinned ancestors living on the African savannah and modern-day Albinos living in central Africa. If the evolutionary fitness of modern albinos suffers because of skin cancer, the problem likely would have been even worse for our ancestors, who didn't benefit from protective clothing, shelter, or any medical advances. So perhaps, skin cancer was a selective factor in the evolution of pigmentation after all. It may have been inappropriately discounted because most of the previous data were focused on skin cancer rates and deaths among lightly pigmented people living in areas that receive less of the sun's radiation.

More research will be required to untangle all the potential reasons that natural selection favored dark skin in our ancestors, but whatever the reasons, we can be sure that it did&mdashand that as modern humans fanned out across the globe, they experienced a wide variety of environments that favored many different skin tones. Today, we see evidence of this complex evolutionary history in both our genes and our unique appearances.


Tanning with p53

The DNA damage caused by UV exposure has long been known to up-regulate p53. Mice that lack p53 have a propensity to develop tumors upon UV exposure. The team now finds that these mice also fail to tan.

Tanning occurs when keratinocytes make more melanocyte-stimulating hormone (MSH) and thus induce melanocytes to produce large amounts of the pigment melanin. MSH is a cleavage product of the POMC pro-hormone. The team found that p53 directly binds to, and increases transcription from, the POMC gene promoter in response to UV treatment.

p53 also promoted POMC and melanin production when induced by factors other than UV, such as the cancer drug etoposide. Melanin provides protection to the skin by mopping up free radicals and by acting as a direct shield from UV radiation. Inducing melanin via the p53 pathway might potentially provide a sunless golden tan. This strategy might be good for reducing cancer risk, although vitamin D levels may need supplementing if the sun were continuously avoided.


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 7).

Figure 7. The relative coloration of the skin depends of the amount of melanin produced by melanocytes in the stratum basale and taken up by keratinocytes.

Melanin occurs in two primary forms. Eumelanin, the most common form of melanin, exists as black and brown, whereas pheomelanin provides a red color. 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 protects the DNA of epidermal cells from UV ray damage and the breakdown of folic acid, a nutrient necessary for our health and well-being. In contrast, too much melanin can interfere with the production of vitamin D, an important nutrient involved in calcium absorption. 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.

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 8).

Figure 8. Moles range from benign accumulations of melanocytes to melanomas. These structures populate the landscape of our skin. (credit: the National Cancer Institute)

Practice Question

What determines the color of skin, and what is the process that darkens skin when it is exposed to UV light?

Integumentary System

The first thing a clinician sees is the skin, and so the examination of the skin should be part of any thorough physical examination. Most skin disorders are relatively benign, but a few, including melanomas, can be fatal if untreated. A couple of the more noticeable disorders, albinism and vitiligo, affect the appearance of the skin and its accessory organs. Although neither is fatal, it would be hard to claim that they are benign, at least to the individuals so afflicted.

Figure 9. Individuals with vitiligo experience depigmentation that results in lighter colored patches of skin. The condition is especially noticeable on darker skin. (credit: Klaus D. Peter)

Albinism is a genetic disorder that affects (completely or partially) the coloring of skin, hair, and eyes. The defect is primarily due to the inability of melanocytes to produce melanin. Individuals with albinism tend to appear white or very pale due to the lack of melanin in their skin and hair. Recall that melanin helps protect the skin from the harmful effects of UV radiation. Individuals with albinism tend to need more protection from UV radiation, as they are more prone to sunburns and skin cancer. They also tend to be more sensitive to light and have vision problems due to the lack of pigmentation on the retinal wall. Treatment of this disorder usually involves addressing the symptoms, such as limiting UV light exposure to the skin and eyes. In vitiligo, the melanocytes in certain areas lose their ability to produce melanin, possibly due to an autoimmune reaction. This leads to a loss of color in patches (Figure 9). Neither albinism nor vitiligo directly affects the lifespan of an individual.

Other changes in the appearance of skin coloration can be indicative of diseases associated with other body systems. Liver disease or liver cancer can cause the accumulation of bile and the yellow pigment bilirubin, leading to the skin appearing yellow or jaundiced (jaune is the French word for “yellow”). Tumors of the pituitary gland can result in the secretion of large amounts of melanocyte-stimulating hormone (MSH), which results in a darkening of the skin. Similarly, Addison’s disease can stimulate the release of excess amounts of adrenocorticotropic hormone (ACTH), which can give the skin a deep bronze color. 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 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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