Go Back   Freethought Forum > The Library > Articles & Essays > Science

Article Tools Display Modes
Intro to Anatomy 7: The Integumentary System
Intro to Anatomy 7: The Integumentary System
The Lone Ranger
Published by The Lone Ranger
Default Skin and Hair Color

Skin and Hair Color:
Factors Determining Skin Color:
Overall, skin color is largely determined genetically. There are at least seven sets of genes that influence skin color, which is why our species shows such a wide range of skin colors. Of course, environment factors also play a major role in one’s skin color.

There are two major types of melanin found in the skin (and hair) of humans. Eumelanin gives the skin and hair a dark brown to black color, and dark-skinned people have relatively large amounts of it. Pheomelanin gives the skin and hair a yellowish to reddish color, and is more abundant in fairer-skinned people. The density of melanocytes in the skin does not seem to vary by skin color, but the melanocytes of dark-skinned people are more active and so secrete more eumalanin and pheomelanin than do the melanocytes of light-skinned people.

There is a third pigment found in epidermal cells, and that is the yellow-orange pigment carotene. Carotene is the same pigment that makes carrots orange (hence its name), and some people actually seem to find it addictive. There are documented instances of people eating so many carrots that their skins turned orange as a result.

The ultraviolet radiation in sunlight can be very damaging to organic molecules (for instance, if it damages DNA in skin cells, it can cause them to become cancerous), and the primary function of melanin in the skin is to absorb UV radiation, thus preventing the radiation from doing harm to tissues. Exposure to ultraviolet radiation causes melanocytes to increase melanin production, and so your skin darkens. This, of course, is what happens when you get a suntan.

Particularly in light-skinned individuals, the blood also plays a role in skin color. For example, when you’re overheated, blood flushed into the skin from deeper in the body causes the skin to become redder. When you’re cold, the opposite occurs; blood is withdrawn from the skin into the body core, causing the skin to become paler.

Anger can also trigger flushing of blood to the skin, causing it redden noticably. Flushing of blood to the skin can also happen when you’re embarrassed – this is blushing. When you’re truly frightened, the opposite happens; blood is shunted away from “nonessential” organs like the skin and the digestive organs and to the skeletal muscles in preparation for “fight or flight.” This causes your skin to become noticably paler.

A cyanotic infant.
This little girl has a defective heart, so the blood
cannot deliver enough oxygen to her tissues.
When blood is well-oxygenated, it is bright red, but when blood oxygen content is low, the blood is dark reddish-purple in color. (Deoxygenated blood is not blue, despite what a lot of people seem to think.) The skin of a person whose blood oxygen content is very low takes on a bluish hue, and the person is said to be cyanotic (from the Greek kyanosis – “dark blue”). Choking victims often develop cyanosis, as they can’t draw oxygen into their lungs and distribute it to body tissues. Cyanosis is often a symptom of heart or circulatory problems, because the blood doesn’t deliver enough oxygen to body tissues, especially those in the extremities.

People who’re very cold generally develop cyanosis in the extremities (including the lips), because blood is withdrawn to the body core in an attempt to reduce heat loss as much as possible.
Variations in Skin Pigmentation:
There are lots of ways that skin pigmentation can vary between two individuals, even if they’re very closely related. These pigmentation differences may be subtle, or they may be enormous.

Freckles are small pigmented spots that appear on the skin of light-skinned individuals. Freckles typically have irregular borders and represent patches of skin with unusually high melanocyte activity compared to the rest of the skin. Exposure to sunlight seems to stimulate the development of freckles.

She has probably spent a lot of time out in the sun.

Lentigos are similar to freckles, but they have regular borders and contain abnormal melanocytes. Senile lentigos or liver spots are dark patches that develop on the skin of older caucasions, probably as a result of long-term exposure to sunlight.

Sometimes clusters of melanocytes and other epidermal cells form a non-cancerous growth known as a nevus or mole. A nevus is quite benign, but a “mole” that appears suddenly, changes color, and/or bleeds may, in fact, be cancerous and should be examined by a physician immediately.

This woman has vitiligo.
An albino (from the Latin albus – “white”) is a person who, because of a genetic defect, cannot produce melanin. Such a person has melanocytes, but since they produce no melanin, the skin is white (pink or red where it’s thinner and blood vessels can be seen through it), as is the hair.

A similar condition to albinism is called vitiligo. In vitiligo, melanocytes in large patches of skin are largely or completely destroyed. It’s still not known what causes vitiligo, but sufferers have large, irregularly-shaped patches of white, pink, or red skin. (Where the unpigmented epidermis is thick-enough to be opaque, it looks white; where the epidermis is thinner, underlying blood vessels can be seen beneath it, giving the skin a pink to red coloration.) In some patients, the melanin-less skin patches are inflamed, suggesting that at least some cases of vitiligo are caused by exposure to noxious chemicals or to infectious agents.

There’s widespread suspicion that Michael Jackson suffers from vitiligo, and that he uses makeup and/or skin-bleaching to hide it.

Jaundice (from Old French jaunice – “yellowness”) occurs when the liver is unable to excrete bile. This causes a yellow pigment to build up in body fluids. In severe cases, the skin and whites of the eyes turn yellow.

Some tumors affecting the pituitary gland cause the secretion of large amounts of melanocyte-stimulating hormone (MSH), which, as its name implies, stimulates melanocyte activity. This causes a darkening of the skin, as if the afflicted person has an extremely deep tan.
Factors Determining Hair Color:
In humans, hair color is determined by the amount and kind of melanin deposited into hair cells by melanocytes in the hair follicle. This, in turn, is genetically determined. There are at least two sets of genes that determine your hair color. One set determines whether you have light hair or dark hair. The gene for light (blond) hair is recessive, so natural blond(e)s must inherit genes for blond(e)ness from both parents. The other gene set determines whether or not you have red hair. (The gene for red hair is recessive and also rather rare. Redheads, like blonds, must inherit the trait from both parents.)

People with naturally blond(e) hair have relatively large amounts of phaeomelanin in their hair and relatively little eumelanin. Blond(e)s have the thinnest hairs but have the greatest density of hair follicles; the average blond(e) has about 140,000 hairs on his or her head. People with blond(e) hair tend to have fair skin and light eyes, though the skin, eye, and hair colors are all determined by different sets of genes.

Redheads have the largest amounts of phaeomelanin in their hair of any hair color and the lowest amounts of eumelanin. Redheads have the lowest density of hair follicles of any hair color (the average redhead has only 90,000 or so hairs on his or her head), but the thickest hairs. Like blond(e)s, redheads tend to have fair skin and light eyes.

The great majority of people worldwide have hair that’s some shade of brown. People with brown hair have lots of eumelanin and relatively little phaeomelanin in their hair cells. People with brown hair tend to have medium-thick strands of hair, and average about 100,000 hairs on their heads. Brown hair tends to be associated with darker-colored skin and eyes.

Black hair is very common among people of African, Asian, and Native American descent. People with naturally black hair have a great deal of eumelanin and little or no phaeomelanin in their hair. In terms of its structure and density, black hair is very similar to brown hair.

As a general rule, there’s a correlation between a person’s ancestry and his or her hair, eye, and skin color. People who trace their ancestry to equatorial regions (where sunlight is more intense) tend to have darker hair, skin and eyes. It’s widely believed that these are adaptations to prevent overheating and to prevent damage to body tissues from the ultraviolet radiation in sunlight.

The melanin in dark-colored skin absobs ultraviolet radiation very well, and so provides excellent protection against skin cancer. Since ultraviolet radiation can cause the development of cataracts in the eyes, having brown or black irises provides protection against cataracts. Dark-colored hair absorbs solar energy very well and helps to protect the brain from overheating as well as from UV radiation.

People whose ancestors hail from more northerly regions tend to have lighter hair, skin and eyes than do people of equatorial ancestry. Lighter skin is probably an advantage in more northern regions because it allows for efficient production of vitamin D where there’s less sunlight than in the tropics. (Dark-skinned people living in northern areas often suffer from vitamin D deficiency.) Loss of the dark pigments in the eyes could be an advantage in more northern regions, because lighter-colored eyes would mean more light reaches the retina of the eye. There’s no definitive test of this that I’m aware of, but there have been some studies claiming that the average light-eyed person can see better in low-light conditions than can the average dark-eyed peron.

Production of Vitamin D:

The skin produces Vitamin D when exposed to UV-B light.
Though exposure to strong sunlight can damage the epidermis and deeper tissues, limited exposure is quite beneficial. When exposed to ultraviolet radiation, cells in the epidermis convert a cholesterol-related steroid to vitamin D or cholecalciferol. Vitamin D is essential for proper development of the bones.

The liver converts cholecalciferol into calcidiol and stores any excess until it’s needed. The kidneys use calcidiol to synthesize the hormone calcitriol. Calcitriol is essential for normal absorption of calcium and phosphorous by the small intestine. If calcitriol isn’t present in adequate amounts (because not enough cholecalciferol is being made by the skin, because the liver cannot convert it to calcidiol, or because the kidneys cannot convert calcidiol to calcitriol), insufficient calcium or phosphorous will be available for proper development of the bones. This causes the condition known a rickets when it afflicts children. A child with rickets has bones that are so soft they bow outward from his or her weight. The condition is known as osteomalacia in adults, but it’s essentially the same thing.

In addition to its essential role in growth and repair of bone, vitamin D is thought to have anti-cancer properties. There is some evidence that vitamin D deficiency makes you more vulnerable to several forms of cancer, including breast cancer, ovarian cancer, colon cancer, and prostate cancer.

The ultraviolet radiation necessary for vitamin D synthesis (specifically, UV-B) only reaches the Earth’s surface in much abundance for a few hours a day when the sun is high. Much less of it reaches the Earth’s surface at high latitudes than at low latitudes, and very little reaches the Earth’s surface on cloudy days or during the winter. Even so, the average fair-skinned person can make and store several days’ worth of vitamin D with just one hour’s exposure to the midday sun. It probably says something about how much of an indoor society we’ve become that vitamin D deficiency is so common in the U.S. that we add it to milk. (Dark-skinned people living at high latitudes are much more likely to suffer from vitamin D deficiency than are light-skinned people, so African-Americans are especially vulnerable.)

These women are diligently working to build strong bones and teeth.
They’re also hoping to reduce their risk of developing ovarian cancer.


Article Tools

Featured Articles
<<  <    Next Page: Integumentary System Disorders (Page 7 of 8)    >  >>
Thanks, from:
Ensign Steve (12-13-2008)

  Freethought Forum > The Library > Articles & Essays > Science

Currently Active Users Viewing This Article: 1 (0 members and 1 guests)
Article Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump


All times are GMT +1. The time now is 10:34 AM.

Powered by vBulletin® Version 3.8.2
Copyright ©2000 - 2024, Jelsoft Enterprises Ltd.

Article powered by GARS 2.1.8m ©2005-2006
Page generated in 0.27150 seconds with 14 queries