Intro to Anatomy 7: The Integumentary System

[The Lone Ranger]

An Introduction to Human Anatomy and Physiology
Chapter Seven: The Integumentary System

Introduction: Where Have All the Frogs Gone? Over the past 50 years or so, biologists have noted a sharp decline in the size of amphibian populations throughout the world. The numbers of frogs, toads, salamanders, and newts appear to be decreasing at an alarming rate, and several species have gone extinct.

The obvious culprit would seem to be loss of habitat due to human activities. Amphibians, after all, are dependent on wet (or at least moist) habitats, and every time a marsh or swamp or pond is destroyed in order to put up a new housing development or shopping mall, the local amphibian populations suffer. But amphibian populations are declining in protected areas too, such as national parks and nature reserves. Why is this?

There are doubtless several reasons, but one very important factor has to do with the nature of amphibians’ skins.

The first vertebrates to live on land were animals that we’d call amphibians. Modern amphibians, presumably like their ancient ancestors, have relatively poorly-developed lungs compared to reptiles, birds, or mammals. In fact, some land-dwelling salamanders lack lungs entirely as adults. So, amphibians absorb oxygen largely (in some cases, entirely) across their skin surfaces. They can do this because their skins are thin, moist, and permeable.

http://www.freethought-forum.com/images/anatomy7/frog.jpg
The Southern Gastric-Brooding Frog (Rheobatrachus silus).
Native to the rainforests of southeastern Queensland,
Australia, the species was last seen in the wild in 1981.
The last-known captive specimen died in 1983, so the
species is presumed to be extinct.
No one knows exactly why.



The downside of having such thin, permeable skin is that it offers very little protection. Water readily crosses amphibians’ thin skins, so they will quickly dehydrate and die in arid environments. Similarly, most amphibians are quickly killed by exposure to salt water. (Just as an aside, this inability to tolerate exposure to saltwater implies that amphibians are almost certainly descended from ancient freshwater fish, not marine fish. The popular notion that the first land-dwelling vertebrates crawled out of the ocean is almost surely false. Both the fossil record and the physiology of modern amphibians strongly suggest that the first land-dwelling vertebrates crawled out of ancient swamps and ponds.)

Their thin, permeable skins also make amphibians very sensitive to air- and water-borne pollutants. Perhaps the frogs and salamanders are like the canaries miners once used to alert them of toxic gases. When the canaries (which are more sensitive to these toxic gases than are humans) fell off their perches, the miners knew that they had to get out of the mine fast. Maybe we should be concerned that the frogs and salamanders are trying to tell us something.

Something like 350 million years ago, the amphibians gave rise to another lineage, the amniotes. The amniotes have quite a few features that distinguish them from the amphibians, but the particular feature we’re concerned with here is that amniotes, unlike their amphibian ancestors, have relatively thick, water-proof skins.

The first amniotes were animals that we’d call reptiles today, and from them descended modern reptiles, birds, and mammals. With their relatively thick, waterproof skins, amniotes can survive and even thrive in environments that are much too harsh for even the hardiest of amphibians.

An amphibian's skin contains mucous glands that help keep the skin moist and lubricated, and poison glands that help to deter would-be predators. In some species, chromatophores in the skin allow them to change their skin color. But that’s really about it.

The skins of reptiles and birds, by contrast, are much more complex. Reptiles and birds have relatively thick skins covered by protective scales made mostly of the protein keratin. (Birds’ feathers are modified scales, and most of the feather-free body surfaces – the feet in most species, for instance – are covered by scales.) The scales of reptiles and birds (and some mammals) are not the same thing as the scales of fishes, by the way. The scales of most fishes are modified bone (in sharks, they’re modified teeth), and they form from completely different tissues than do the scales of amniotes.

Of all the amniotes, though, it is mammals such as ourselves that have the most complex skins. Mammals’ skins are underlain by an insulating layer of fat that helps to conserve body heat even in very cold environments, and the skin itself contains many different kinds of specialized glands that aren’t found in other amniotes.


Skin and the Integumentary System:http://www.freethought-forum.com/images/anatomy7/selleck3.jpg
Tom Selleck shows off his largest organ.
As you recall, two or more kinds of tissues grouped together and performing a common, specialized function make up an organ. Thus, the membrane that covers the body surface – the cutaneous membrane or the skin – is an organ. In terms of its surface area, it’s certainly the largest organ of the body. The cutaneous membrane, along with various accessory organs, makes up the integumentary system. (From the Latin integumentum – “a covering.”)

Skin (The Cutaneous Membrane): The skin provides protection against external threats, including invasion by pathogens. It helps to regulate the body temperature, and it prevents water loss. The skin houses sensory cells that allow us to detect changes in our external environments. The skin also synthesizes certain important molecules, including vitamin D, which is not only important in building strong bones and teeth, but seems to have anti-cancer properties. (Some studies suggest that moderate exposure to sunlight can lower your risk of developing certain cancers, including breast, colon, and prostate cancers, because of increased vitamin D production. Of course, too much exposure to sunlight increases your risk of skin cancer.)

The skin consists of two layers, the epidermis and the dermis. The outermost layer, the epidermis, consists of stratified epithelium. (An acquaintance of mine likes to walk up to unsuspecting people and say, “Your epidermis is showing.” He finds it vastly amusing to watch them frantically check their buttons and zippers.) The dermis is deep to and thicker than the epidermis; the non-living basal lamina separates the epidermis from the dermis. The dermis contains fibrous connective tissue, epithelium, smooth muscles, nervous tissue, and blood vessels.

Beneath the skin proper lie masses of loose connective and adipose tissues. These tissues bind the skin to the underlying organs and make up what is known as the subcutaneous layer or hypodermis.

http://www.freethought-forum.com/images/anatomy7/skin.jpg
The cutaneous membrane and the hypodermis.
The hypodermis (subcutaneous layer) is not actually part of the skin.


[B]The Epidermis:
The epidermis (epi – “above” + dermis – “skin”) consists of stratified squamous epithelium and contains no blood vessels. The deepest layer of cells in the epidermis, however, the stratum basale (basale = “base” stratum = “layer”), is close to blood vessels in the underlying dermis, so those cells are well-supplied with nutrients.

The cells of the stratum basale grow and divide rapidly, and as new cells are produced, older cells get pushed upward. As the older cells are pushed away from the blood vessels in the dermis, their nutrient supply decreases and they eventually die, which means that the outermost portion of the epidermis is not living tissue.

As the older cells, called keratinocytes (keratino – “keratin-producing” + cyte – “cell”) are pushed upward, they undergo a process called keratinization. This occurs when strands of the tough, waterproof protein keratin are deposited into the cytoplasm of these cells. Keratinization of epidermal cells means that the outermost portion of the epidermis, the stratum corneum (corneum = “horny”) is made up of dead, heavily keratinized cells that form a tough, waterproof protective layer. These cells are constantly being shed or abaded off and replaced by new cells coming up from the stratum basale. In fact, it’s often claimed that most of the “dust” in your house actually consists of shed epidermal cells.

Normally, production of epidermal cells by the stratum basale balances loss of cells from the stratum corneum. Where the skin is subjected to frequent pressure or abrasion, the rate of cellular production increases in the stratum basale, causing the epidermis in these regions to become thicker – and so calluses form.

Specialized cells within the stratum basale known as melanocytes produce the pigment melanin, which they can then transfer to other epidermal cells. Melanin can be yellow, brown, or black in color, and the more of it that is produced by melanocytes, the darker is the skin.

http://www.freethought-forum.com/images/anatomy7/melanocyte.jpg
A melanocyte in the epidermis.
Melanocytes have projections that extend outward and between other epidermal cells. With these
cellular projections, melanocytes can deposit the melanin they produce into other epidermal cells.
This is what gives the skin its color.


The Dermis: http://www.freethought-forum.com/images/anatomy7/cleavage.jpgLines of Cleavage in the skin.
A good surgeon cuts along these lines, not across them.
The dermis binds the epidermis to underlying tissues. It consists largely of fibrous connective tissue that contains many collagen and elastin fibers. This makes the dermis quite strong, but very elastic as well. Blood vessels in the dermis supply nutrients and oxygen to all the cells of the skin. These blood vessels also play a major role in regulating body temperature, which we will discuss in a bit. Various accessory organs are embedded in the dermis, including hair follicles, sebaceous glands, and sweat glands.

The dermis contains many nerve fibers. These include motor fibers that carry impulses from the brain and spinal cord to muscles and glands in the skin. There are also many sensory fibers in the dermis that carry impulses from the skin to the brain and spinal cord. These fibers give us the ability to feel touch, pain, heat, and cold.

Fibers of the proteins collagen and elastin extend throughout the dermis. These fibers give the skin strength and allow it to stretch without tearing. Of course, if these fibers are stretched too much, they’ll lose their ability to return to their original shapes. Distortion of the dermis that occurs during pregnancy or after extensive weight gain can stretch collagen and elastin fibers beyond their ability to recover. The resulting damage to the dermis creates wrinkles and creases in the skin known as stretch marks.

Collagen and elastin fibers are generally arranged in parallel bundles within the dermis and oriented such that they can most efficiently resist the stress that normally occurs on the skin during movement. The pattern of protein fiber bundles establishes the lines of cleavage of the skin. These lines of cleavage are of utmost importance to surgeons, because a cut made parallel to a line of cleavage will usually remain closed with only minimal bleeding. Such a cut heals with a minimum of scarring. By contrast, a cut made at a right angle to a cleavage line will be pulled open as cut elastin fibers recoil. This cut will bleed profusely and will produce much scar tissue as it heals.

Needless to say, any competent surgeon will normally want to cut along the lines of cleavage, rather than across them.


[B]The Subcutaneous Layer (Hypodermis): The hypodermis (hypo – “under” + dermis – “skin”) or subcutaneous (sub – “below” + cutaneous) layer lies below the skin and is made up of loose connective and adipose tissues. There is no distinct boundary between it and the lower portion of the dermis.

The hypodermis binds the skin to underlying organs while allowing the skin to move somewhat independently of underlying structures. Adipose tissue in the hypodermis provides padding and shock-absorption that helps to protect underlying tissues from damage; it is also important in insulating against loss of body heat. Because the subcutaneous layer contains numerous blood vessels but no vital organs, it is a near-ideal place to inject drugs. This is why so many drugs are administered through subcutaneous injection by a hypodermic (hypo – “under” + dermic – “skin”) needle.

Babies and young children have extensive deposits of “baby fat” in the hypodermis, which helps provide additional insulation against heat loss. This is important for young children, because the smaller a warm-blooded animal is, the faster it tends to lose body heat across its skin surface. As children grow larger and become less vulnerable to heat loss, these fat layers (hopefully) become thinner.

Of course, even adults have considerable amounts of adipose tissue in the subcutaneous layer. Interestingly, body fat tends to be distributed differently in men and women. In men, subcutaneous fat accumulates primarily in the neck, arms, and lower back, above the buttocks, and in the abdominal region (the “paunch”). Subcutaneous fat tends to be more evenly distributed in women. Women generally have proportionately more body fat than do men, and their subcutaneous layers are generally thicker – this is one reason why women tend to have softer skin than do men. In women, subcutaneous fat is especially prone to accumulate in the breasts, buttocks, hips, and thighs.

Most marine mammals have thick layers of subcutaneous fat (called blubber) that provide insulation against loss of body heat to the surrounding water. Because women tend to have thicker subcutaneous layers than do men, this means that they’re better insulated against heat loss in some ways. (This tendency may have evolved because women, being smaller than men generally, are more vulnerable to heat loss.) There are some interesting consequences of this.

Perhaps you’ve heard of the Donner Party? The party consisted of 89 people. During the winter of 1846/1847, 81 members of the Donner Party were trapped by snow while trying to cross the Sierra Nevada Mountains (several people died before the party actually became trapped), and they soon ran out of food. Ultimately, 41 people died. The interesting thing is that 2/3 of the men died while 2/3 of the women survived. Why?

As a general rule, men have higher metabolic rates than do women, and so they produce more body heat. So long as enough food is available to keep their internal fires going then, men tend to feel the cold less than women do. On the other hand, being better insulated on average, a woman will lose body heat less rapidly than will a man of the same size. What’s more, since the average woman has more stored body fat than does a man of the same size, she has more “fuel” to survive on than does the average man.

This means that the average woman will survive longer than the average man under starvation conditions. That’s especially true when it’s cold-enough for hypothermia to be a serious concern.

In the cold waters off the coasts of Korea and Japan, divers known as ama have been making their livings diving for shellfish for centuries. They dive to 30 meters (100 feet) or more without diving equipment, and stay underwater for 2 minutes or longer. Traditionally, an ama wears either a simple loincloth or no clothing at all while diving. (In modern times, the ama have become a tourist attraction; where tourists are likely to be watching, the ama generally wear thin cotton garments.) The word “ama” translates as “sea woman”; in Korea, all ama are women, though some Japanese ama are male.

Why are the ama almost always women? It’s thought that women, since they’re better-insulated than are most men, are less prone to loss of body heat, and so are less susceptible to hypothermia.

As an aside, the ama traditionally hyperventilate themselves and then give a low whistle just before plunging into the water. Though it’s doubtful that many of the ama know what functions the whistle serves, it is nonetheless important, because it does two vital things.

The deeper you dive, the more pressure the surrounding water exerts on your body. Some ama dive so deeply that their lungs are compressed to less than 2/3 of their volumes at the surface. When they whistle just before diving, the ama empty some of the air from their lungs. The delicate tissues inside the lungs simply aren’t equipped to handle the internal pressure that would be exterted by a lungful of air compressed to 2/3 of its original volume, and so diving to that depth with your lungs completely inflated could cause serious injury.

The whistling has a subtler function as well. You’re surely aware that if you hold your breath (without first hyperventilating yourself) for long enough, you’ll eventually be compelled to take a breath. Most people are under the impression that the urge to take a breath is triggered by low blood oxygen levels, but that’s incorrect. It’s high blood CO2 levels that are responsible. So what?

Well, hyperventilation before diving, does not raise your blood O2 levels significantly, contrary to what most people think. What hyperventilation does is lower your blood CO2 levels. Since it’s high CO2 levels that trigger the breathing response, hyperventilation can indeed increase the length of time that you can hold your breath, but it’s a very dangerous thing to do when diving. If you hyperventilate before a long dive, you can exhaust your available oxygen supply while underwater without realizing it, because the blood CO2 levels never rise to a high-enough level to cause you to feel the urge to breathe. When the brain runs out of oxygen, you lose consciousness. Needless to say, if you happen to be underwater at the time, the result will probably be fatal.

The long, low whistle the ama make just before diving has the effect of preventing them from blowing off too much CO2, and so greatly reduces the likelihood that they’ll lose consciousness while beneath the surface and drown.

http://www.freethought-forum.com/images/anatomy7/ama.jpg
The ama have been diving for pearls, seaweed, and shellfish in the cold waters off Japan and Korea for centuries.


Accessory Organs of the Skin: The accessory organs of the skin include hair follicles, sebaceous glands, sweat glands, and nails. In the developing embryo, these structures originate from epidermal tissues, even though many of them penetrate down into the dermis or even into the subcutaneous layer by the time of birth.

[B]Hair Follicles and Hair:
http://www.freethought-forum.com/images/anatomy7/demodex_folliculorum.jpg
Demodex folliculorum, seen with an electron microscope.
Isn’t it cute? Wouldn’t you like to have 2 or 3 or a few
hundred as pets? Don’t worry: chances are, you already do.

A hair follicle consists of epidermal tissue that forms a tube which plunges down into the dermis and sometimes even into the subcutaneous layer. Each hair follicle produces a hair shaft. There are about 5 million hair follicles on the average person’s body – only 2% of which are on the head. The only portions of the body surface that lack hair follicles are the sides and soles of the feet, the palms of the hands, the sides of the fingers and toes, the lips, and portions of the external genitalia.

Here’s an interesting factoid. There’s a species of small mite known as Demodex folliculorum that lives in the hair follicles of your eyelashes and eyebrows, and sometimes elsewhere on the body. They’re perfectly harmless, and subsist on dead skin cells and skin secretions. About 98% of us have them.

Hair Production: The cells of a hair follicle produce a hair in very much the same way that the stratum basale produces the cells of the epidermis. Epithelial cells near the base of the hair follicle divide to produce the cells that will make up the growing hair. As the cells are pushed upward, they become compressed and very heavily keratinized. So, hair is made of the same sort of cells that make up the stratum corneum of the skin, only the hair cells are much more densely-packed and much more heavily keratinized. Since hair cells die long before they reach the surface of the skin, hair, like the stratum corneum, is non-living tissue.

Cells in the central portion of the hair (the medulla or core of the hair) contain soft keratin, which makes the central portion of a hair flexible. In many mammals, the core of a typical hair is hollow, which makes it an excellent insulator against heat loss. Hollow hairs are also more bouyant, which can be important for aquatic mammals. (Porcupines, of course, take this to an extreme; their quills are modified hairs – very thick, but hollow.)

Cells in the outer portion of the hair (the cortex) contain hard keratin. This makes the outer portion of a hair much stiffer than the inner portion.

Hair follicles that are hook-shaped near the base tend to produce curly hairs. Keratin makes up the bulk of a hair, and keratin, like many proteins, contains lots of disulfide bonds, which are strong covalent bonds between nearby sulfur atoms. It’s thought that the “hooked” hair follicles deposit more sulfur compounds on one side of a hair than the other, which causes the hair to pull into a curl. Some chemical curling agents work by breaking the disulfide bonds (heat can also be used to break the disulfide bonds). If the disulfide bonds in a straight hair are deliberately broken, the hair is wrapped around a curling device, and then a chemical is added that reconstitutes the disulfide bonds, the hair will remain curled when the curlers are removed. This is what happens when you get a “permanent.”

Straight hair follicles tend to produce straight hairs, perhaps because they deposit sulfur compounds evenly in the hairs they produce. Just as heat and/or chemicals can be used to curl straight hair, they can be used to straighten curly or wavy hair.

The genetics of straight/wavy/curly hair seem to be very simple and straightforward since there seem to be only two genes involved, and neither is dominant over the other. If you inherit a gene for “curly” hair from each parent, you’ll have curly hair. Similarly, if you inherit a gene for “straight” hair from each parent, you’ll have straight hair. If you inherit a “straight” gene from one parent and a “curly” gene from the other, you’ll have wavy hair.

Functions of Hair: The roughly 100,000 hairs on your head protect your scalp from ultraviolet light, help cushion a blow to your head, and insulate your skull. Most of the body heat you lose on a cold day is from your head, and the brain is the one organ in your body that’s most sensitive to overheating. Scalp hair is therefore doubly important, because it helps insulate the brain against heat gain on hot days, just as it insulates against heat loss on cold days.

Hairs in the nostrils and the ear canals help prevent the entry of foreign particles or insects. (Not perfectly, though; when I was 12, a moth flew into my ear and I had to be taken to a physician to have it removed.) The lashes of the eye help prevent foreign objects from entering the eye – camels, living in areas where there’s lots of blowing dust and sand, have very long eyelashes. The eyebrows help to reduce the likelihood that sweat dripping from the forehead will fall into your eyes.

Because the base of each hair follicle is surrounded by sensory nerve fibers, you can feel the movement of even a single hair shaft. This acts as an early-warning system that may help to prevent injury. For example, you may be able to swat a mosquito before it reaches your skin surface. The whiskers of cats and most other mammals are simply elongated and stiffened hairs, and they help the animals navigate in the dark.

Hair in the armpits and groin serves at least two functions. For one thing, it provides lubrication so that the skin under the arms and between the legs doesn’t abrade when we walk. It also has the effect of trapping pheromones, giving each of us a distinctive odor. Many studies have shown that we can detect and respond to these chemicals, even if we’re not consciously aware of it. (These chemicals seem to be important in mate selection, for instance. Studies have shown that women can determine whether a given man is a close relative or not by his body odor – and women consistently prefer the smell of men who are not close relatives. When we talk about two people having the “right chemistry,” we may be more correct than most of us would ever dream.) Of course, if you wear clothing, the clothes trap those volatile chemicals; if you don’t bathe frequently enough, those trapped chemicals begin to decompose into compounds that are somewhat less pleasant-smelling.

Each hair follicle has a smooth muscle attached to it called the arrector pili (arrector = “erector” pili = “hairs”). When stimulated, the arrector pili contracts, pulling the hair shaft straight. This produces “goosebumps,” and it can happen when you’re frightened or angered, or when you’re cold. Most mammals erect their fur when frightened, which makes them look larger and more dangerous to a would-be attacker. Erecting the fur when cold makes for a deeper layer of insulation against heat loss.

Although a human’s “fur” isn’t thick enough to provide much insulation, we still retain the vestigial trait of erecting the hair when frightened, angry, or cold.

Types of Hairs: There are two major types of hairs in adults, vellus hairs and terminal hairs. Vellus hairs are the fine “peach fuzz” hairs scattered over most of the body surface. Terminal hairs are heavy, more deeply pigmented, and usually longer. Sometimes, terminal hairs are curly. The hairs of your head, including the eyebrows and eyelashes, are terminal hairs; so are the axillary hairs in your armpits and the pubic hairs in your groin.

At puberty, rising levels of testosterone and other male hormones cause vellus hairs on some parts of the body to be transformed into terminal hairs. These hairs that transform from vellus to terminal when stimulated by male hormones are called androgenic (andro – “male” + genic – “created”) hairs. Both males and females produce testosterone, though males produce much more of it, of course. (Similarly, both males and females produce estrogen, but females produce much more of it.) In both sexes, rising hormone levels cause vellus hairs in the armpits and groin to convert to terminal hairs. The higher testosterone levels in males typically cause much of the vellus hair of the face and chest (and in some cases, the back) to convert to terminal hairs as well.

Since testosterone is a steroid, use of anabolic steroids by athletes can, among things, promote increased conversion of vellus hair to terminal hair. (This can be somewhat embarassing if the athlete in question is a female, I should think.) Unusually “hairy” men (or women) are said to be hirsute. They don’t have more hair follicles than other people, it’s just that more of their vellus hairs have been converted to terminal hairs.

Developing embryos are covered with a fine coat of hair called lanugo. This hair is shed before birth, and it’s thought to be an evolutionary leftover of sorts – a reminder that we’re descended from significantly hairier ancestors.

Growth and Replacement of Hair: http://www.freethought-forum.com/images/anatomy7/tran_van_hay.jpg
Tran Van Hay
An individual hair grows for only so long, then it is shed. Of course, some hairs grow for longer times than do others, which is why the hairs on your arms and legs aren’t as long as those on your head.

A typical scalp hair grows for 2 – 5 years before it eventually falls out, at a typical rate of about 0.33 millimeters per day. If left uncut, head hairs may grow to 3 feet or so in length, though there’s a great deal of variation in how long these hairs can grow. Some people can grow their head hair to 6 feet or so in length (this typically takes more than a decade). A few individuals have managed to grow their hair to truly astonishing lengths. Supposedly, a Chinese woman named Xie Qiuping has not cut her hair in over 30 years, and it is over 18 feet long. A Vietnamese man named Tran Van Hay claims to have even longer hair. Most people’s hairs don’t grow anywhere near to that length before falling out, however.

Most body hairs grow for 3 – 6 months before growth stops and the hairs eventually fall out. As you might imagine, the hairs in the armpits and pubic region grow for a longer time than do most other body hairs, but not so long as do the scalp hairs.

Forensic Analysis of Hair: Because hair is compressed and keratinized epidermal cells, it will absorb nutrients and other chemicals and incorporate them into its structure as it grows. This means that analysis of hair can provide clues about a person’s health and about what (s)he has ingested. For example, people suffering from lead or arsenic poisoning will have unusual amounts of these metals in their hair.

In 2004, an analysis of some hair taken from King George III of England (who died in 1820) showed very high levels of arsenic. Arsenic poisoning may well have been a contributing factor to his poor health during his later years. While it’s possible he was deliberately poisoned (arsenic was used by assassins for centuries), it’s more likely that he was poisoned by the water he drank or – ironically – by the medicines he was given in a vain attempt to treat his physical and mental problems.

Analysis of hair samples taken from Napolean Bonaparte (died in 1821) also showed very high arsenic levels. Again, this is by no means proof that he was deliberately poisoned, though it’s widely believed that he suffered and ultimately died from arsenic poisoning while imprisoned on Saint Helena Island.

Ludwig van Beethoven suffered chronic illness for most of his adult life and died in 1827 at the age of 57. An analysis of his hair in 2000 showed extremely high lead levels, which would easily account for his symptoms and for his early death. He probably ate from lead utensils, and in so doing, slowly poisoned himself.

Police sometimes test the hair of suspected drug-users for traces of such chemicals as cocaine or marijuana. If a person uses cocaine or other such drugs, traces of it will remain in the hair long after they’ve been flushed out of the rest of the body tissues.

The DNA in hair cells can be analyzed in order to identify individuals or to trace family relationships, though it’s not an entirely reliable process, because the DNA in hair tends to be highly degraded. Trying to get usable DNA from hair is difficult and time-consuming, and there’s always the chance that the samples will be contaminated by “outside” DNA. Nonetheless, it isn’t impossible. (If part of the follicle is still attached to the hair, that’s a great help, because it’s far easier to extract DNA from the unkeratinized cells of a follicle than from the highly keratinized cells of a hair.) Since we’re constantly shedding body hairs, genetic analysis of a hair found at a crime scene can provide convincing evidence that a particular individual was present at the crime scene, though you can’t prove the hair in question belonged to any particular individual. You can say with more or less complete confidence that a particular individual couldn’t have produced the hair in question though, which has sometimes helped clear wrongfully-accused persons.

A typical cell contains much more DNA in its mitochondria than in its nucleus, and so it’s much easier to extract usable amounts of mitochondrial DNA from hair than nuclear DNA. (The downside is that mDNA is much less variable than is nDNA, and so it’s much less useful for distinguishing between individuals.) In 1996, Paul Ware of Tennessee was convicted of rape and murder based on analysis of mitochondrial DNA extracted from a single hair found in the victim’s throat. The mDNA of the hair matched Ware’s mDNA, and that was considered sufficient evidence to convict him.

One of the reasons that it’s possible to do studies like these is because hair is extremely long-lasting. The stuff decays far more slowly than do virtually any other body tissues, and under some conditions hair outlasts even bone.


Aging, Hair Loss, and Pathology: As we age, production of pigment by cells in the hair follicle decreases and the hair lightens, eventually becoming gray or white. Hair is white if it’s unpigmented and it contains air bubbles in the medulla.

The average person loses about 50 hairs from his or her head per day, though there are various conditions that can increase that rate substantially. In males, changes in sex hormone levels with age can cause a shift from terminal hair production to vellus hair production, resulting in male pattern baldness. (It can also affect women, but that’s much rarer.)

Radiation or chemicals used to treat cancer often cause temporary hair loss. This is because anti-cancer treatments typically target all rapidly-dividing cells, not just cancerous cells. Since cells in the hair follicle grow and divide quite rapidly, they’re killed by anti-cancer treatments, just as are cancer cells. (This is also true of the rapidly-dividing cells lining the stomach and intestine, which is one reason why so many anti-cancer therapies cause digestive problems and drastic weight loss.)

Stress, vitamin A overdose, high fever, and hormonal changes during pregnancy are all factors that can cause hair loss. Many people who go on diets to lose weight can also find themselves losing hair if they don’t balance their nutrient intake carefully; dieting can cause drops in levels of iron, zinc, magnesium, and vitamins D, B, and A. Insufficient levels of any of these nutrients can cause hair loss.


http://www.freethought-forum.com/images/anatomy7/hypertrichosis.jpg“Wolfman” Fajardo Aceves Jesus Manuel, of Mexico
He has hypertrichosis.

Hirsutism occurs when a woman experiences “excessive” growth of hair in the same pattern that adult males do. More precisely, hirsutism occurs when vellus hairs on a woman’s face and chest (and sometimes her back) are converted to terminal hairs just as they are in men.

Hirsutism can be triggered by anything that increases a woman’s level of androgens. [Androgens (andro – “man” + gen – “creating”) are the male sex hormones; women produce them too, of course, but normally in much lower concentrations.] An example of a condition that can cause hirsutism is polycystic ovary syndrome. PCOS occurs when a woman’s ovaries don’t produce all the hormones needed for ova to mature. Since the ova don’t mature, ovulation doesn’t occur. Instead, the immature ova develop into cysts that produce androgens. Tumors in the ovaries or adrenal glands can also cause increased androgen production in women and hirsutism.

Some medications change a woman’s hormone balance and can cause hirsutism in sensitive individuals. Birth control pills can do it, for instance. Of course, use of anabolic steroids can cause hirsutism.

Hypertrichosis (hyper – “above” + tricho – “hair” + sis – “condition”) is similar to hirsutism, but it involves growth of terminal hairs on parts of the body where they don’t normally develop even in men. In most cases, hypertrichosis seems to be genetically caused, and it is not typically associated with unusual androgen levels. In severe cases, an afflicted person is covered with a thick coat of fur.

There is some suspicion that the occasional case of hypertrichosis is what inspired legends of such creatures as werewolves. Nowadays, the condition is sometimes called “Wookieeism,” after the Wookiees of Star Wars fame.


[B]Sebaceous (Oil) Glands: Sebaceous (seb – “tallow” or “grease” + aceous – “of or related to”) glands are holocrine glands that secrete a waxy, oily substance called sebum into follicles. The cells that make up sebaceous glands are modified epidermal cells, similar to those that make up hair follicles. In fact, sebaceous glands and hair follicles are intimately related, and most sebaceous glands empty into hair follicles. When the arrector pili muscles contract, they squeeze nearby sebaceous glands and cause sebum to be secreted into the hair follicle and onto the skin.

Even though the cells of the stratum corneum and the hair are dead and keratinized, they dry out and become brittle when exposed to the environment. Sebum serves to lubricate and waterproof the hair and stratum corneum, keeping them flexible. Since soap washes away this sebum, excessive washing of the hair can cause it to become brittle. Excessive washing with soap can cause the skin to become brittle too, leading to cracking and peeling.

Of course, the skin is waterproof only up to a point. If you soak in a bathtub or pool long enough, most of the sebum will be washed away. If this happens, the epidermal cells will begin to absorb water and swell. Because the skin’s surface area has increased but the volume of the body that is covers hasn’t, the skin wrinkles. In most people, the epidermis is thickest on the hands and feet, so this is where most water absorption occurs and therefore where wrinkling is most obvious.

Naturally, after you get out of the bath, water begins to evaporate from your epidermal cells. The cells eventually return to their normal size and the wrinkles disappear.

Sebum contains antibacterial compounds that inhibit bacterial growth and so help protect against infection. So, ironically enough, people who wash themselves frequently in an effort to avoid infection may actually be making themselves more vulnerable to bacterial infection (by removing the protective coating of sebum), not less.

Some sebaceous glands open into follicles that never produce hair, and so the sebum they produce is secreted directly onto the surface of the skin. These sebaceous follicles are especially common on the face, back, chest, nipples, and male genitalia, which is why the skin of these body regions tends to be “oily.”

Sweat (Sudoriferous) Glands: Sudoriferous (sudor – “sweat” + iferous – “bearing”) glands produce sweat. Broadly speaking, there are two different kinds of sweat glands, apocrine sweat glands and merocrine (eccrine) sweat glands.

The apocrine sweat glands, as you’ve surely guessed, are apocrine glands. They are closely associated with hair follicles, and empty their products into hair follicles instead of directly onto the skin. The fatty fluid produced by apocrine sweat glands is sticky, cloudy, and somewhat odorous.

Apocrine sweat glands are especially common in the armpits, around the nipples, and in the groin. These glands don’t become fully active until you reach puberty, and they produce many of the chemicals that give each of us his or her own distinctive body odor. (If you don’t bathe often enough, breakdown of these chemicals by bacteria produces “B.O.”) These glands become more active when you’re excited – frightened, in pain, or sexually aroused – and so there might actually be something to the old notion that dogs and other animals with keen senses of smell can literally “smell fear.”

The merocrine (eccrine) sweat glands are – no surprise here – merocrine glands. The merocrine sweat glands are far more common than are the apocrine sweat glands. They’re distributed over pretty-much the entire body surface; the forehead, the palms of the hands, and the soles of the feet have the highest densities of merocrine sweat glands. Unlike the apocrine sweat glands, merocrine sweat glands secrete their products directly onto the skin surface, instead of into hair follicles.

The sweat produced by the merocrine sweat glands is 99% water, but it also contains some electrolytes (especially sodium chloride, which gives sweat its salty taste), plus metabolic waste products, including urea. (So, sweat has pretty-much the same chemical composition as does urine. Lovely thought for a warm summer day, that.)

Because water absorbs heat as it evaporates, evaporation of sweat produced by merocrine glands removes excess heat from the body. This process is absolutely vital to maintenence of normal body temperature, as temperatures above about 104 degrees Fahrenheit are life-threatening. In a warm environment, even mild exertion will cause a person who cannot sweat for some reason to quickly overheat.

In an arid environment (where sweat evaporates rapidly), so long as a person has enough water, sweating can be an astonishingly effective way to keep cool. In a physiology class, I once saw a film in which a young man stripped down to a pair of shorts and then stepped into a large oven. The only other thing he brought in with him was a raw steak. He sat on a wooden stool in the center of the oven. A pipe carried cool water into the oven so that he could drink. The temperature in the oven was raised to over 300 degrees Fahrenheit and he sat in the oven for 90 minutes, drinking almost constantly. His body temperature never went above 100 degrees. When he stepped out of the oven, the steak was thoroughly cooked.

Sweating doesn’t work nearly that well to prevent overheating in more humid environments, unfortunately, because water doesn’t evaporate as rapidly when the humidity is high.

When your merocrine sweat glands are working at full capacity, your rate of perspiration can exceed a gallon per hour. This rapid loss of water (and to a lesser extent, electrolytes) can be life-threatening, which is why desert hikers and athletes in endurance sports and must be careful to drink plenty of fluids at frequent intervals.


http://www.freethought-forum.com/images/anatomy7/sweating.jpgWhy is this man sweating?
Perhaps he just saw the latest poll numbers?

The merocrine sweat glands, like the apocrine sweat glands, increase production when you’re frightened. This is why the palms of your hands (and the soles of your feet) often become clammy when you’re scared.

The fact that the soles of the feet produce relatively large amounts of sweat is sometimes cited as a partial explanation for why people can walk across hot coals without injury, so long as they do it quickly. This is claimed to be due to the Leidenfrost effect; the sweat forms a protective barrier for your feet. Heat from the coals goes into boiling the sweat instead of heating your foot, and so long as you don’t stay on the coals long-enough for the sweat to completely evaporate, you won’t be burned.

In fairness, the Leidenfrost effect isn’t the only reason you can walk on hot coals without being burned, and it has never been convincingly demonstrated that it plays a significant role at all. The Leidenfrost effect is why you can wet your finger and touch it to a hot iron without getting it burned, however. (It’s also why a drop of water will “dance” across a hot skillet instead of immediately evaporating – a “barrier” of steam forms under the water drop and holds it up above the hot surface, preventing it from evaporating.)

Interestingly, the density of merocrine sweat glands in your skin is largely determined by the environment you experience during your early childhood. People who spend the first few years of their lives in cold climates typically have fewer than half as many eccrine sweat glands per square inch of body surface as do people who spend the first few years of their lives in warm climates.

Mammary Glands: The mammary (from the Latin mamma, meaning “breast”) glands are highly modified apocrine sweat glands contained within the breasts. (Contrary to what a lot of people seem to think, the breasts are not the same thing as the mammary glands; the mammary glands are contained within the breasts.) The mammary glands, of course, are normally active only in females who have given birth, and they produce milk.

Ceruminous (Wax) Glands: Ceruminous (from the Latin cera, meaning “wax”) glands are modified sweat glands found in the external auditory canals of the ears. The secretions of ceruminous glands mix with those of nearby sebaceous glands to produce a mixture called cerumen or ear wax. Cerumen helps to trap foreign particles or small insects and prevents them from reaching the eardrum.

Nails: The nails cover the dorsal surfaces of the fingertips and toetips, providing additional protection for these body surfaces. They’re basically the same structures as the claws of your cat or dog, but flattened instead of rounded and pointed.

The nails are formed by epidermal cells in a manner very similar to the way that hairs are formed. The difference is that nail cells are even more tightly-packed and heavily infused with keratin than are hair cells.

Production of a nail occurs at the nail root, which is an epithelial fold that isn’t visible from the surface. As epithelial cells in the nail root lay down more cells in the growing nail, older cells are pushed outward and the nail lengthens. A portion of the stratum corneum of the nail root extends over the exposed portion of the nail, forming the cuticle or eponychium (epi – “over” + onyx – “nail”).

As the growing nail extends out beyond the eponychium, it slides over the nail bed. The free edge of the nail extends over a thickened stratum corneum, the hyponychium.

The nail itself is more or less translucent, so you can see blood vessels in the nail bed below it; that’s what makes (most of) the nail look pink from above. If the nail suffers a severe blow, some of these blood vessels may rupture and allow blood to collect under the nail and then clot, making the nail look blue or even black.

Near the base of the nail, actively-dividing cells in the nail bed are thicker and obscure the blood vessels beneath. This is why you see a white, half-moon shaped feature called the lunula (from the Latin luna, meaning “moon”) there.

http://www.freethought-forum.com/images/anatomy7/nails.jpg
Anatomy of a fingernail.


Like the hair, the nails incorporate nutrients and other chemicals into themselves as they grow. So, like hair clippings, nail clippings can be used to diagnose certain disorders or to look for evidence of poisoning or drug use.

It’s widely believed that the hair and nails continue to grow for some time after death. This isn’t true. All of the body cells die within minutes after the heart stops, because they’re no longer being supplied with oxygen.

As the body dehydrates after death, the skin shrinks. This exposes more of the hair roots and nail roots than are normally visible, creating the impression that the hair and nails continue to grow for some time after death. This is probably the source of the mistaken belief.



[B]Regulation of Body Temperature by the Integumentary System: Thermoregulation is the process by which an animal controls its body temperature. In mammals such as ourselves, the temperature of the blood is constantly monitored by the hypothalamus of the brain. If the blood’s temperature falls too low, the hypothalamus triggers heating measures, and if the blood’s temperature rises too high, the hypothalamus triggers cooling measures.

Overheating of the blood triggers sweating. It also triggers the opening of blood vessels in the dermis, so more blood flows from the body core (where heat is being generated) to the skin, where the heat can be shed to the outside environment. This is why people who’re overheated flush bright red – it’s because of all the blood being delivered to the skin in an effort to cool it. An overheated person is especially likely to flush red in the face; this is because the brain is so sensitive to overheating, so it’s especially important that blood be cooled before entering the brain. When you’re overheated, an elaborate system of blood vessels pumps blood to the face for cooling before it goes to the brain.

Chilling of the blood triggers closing down of blood vessels in the dermis, so that blood is directed into the body core and beneath the insulative fat layer in the hypodermis. This greatly reduces heat loss to the outside environment. It also explains why fair-skinned people become noticably paler when they’re cold.

Cold also triggers contraction of the arrector pili muscles and erection of the body hair, though this has little effect on heat retention.

In severe cold, rapid, involuntary contraction of the skeletal muscles is triggered. This is known as shivering. Since muscles generate heat as they contract, the rapid muscle contractions can boost heat production significantly.

Injury and Repair of the Skin: Because the cutaneous membrane consists largely of rapidly-growing epithelial cells, it usually heals quickly and completely when damaged. How quickly and completely depends on the wound.

An incision is a slender, straight cut made with a sharp object. An incision typically heals fairly rapidly, since there’s relatively little tissue damage. This is the sort of “wound” a skilled surgeon makes.

A laceration is a jagged cut or tear of the skin. It damages considerably more tissue than does an incision of the same length and depth, and so takes longer to heal.

A scraping wound is an abrasion, and a deep abrasion can cause damage to large amounts of epidermal and dermal tissues. Because of the large area of skin that needs to be repaired, deep abrasions tend to take quite a lot longer to heal than do incisions. A further complicating factor is that abrasions easily become infected, which slows down the healing process even more.

Puncture wounds typically don’t bleed as much as do incisions, lacerations, or abrasions. On the other hand, when the skin is punctured, bacteria and other infectious agents are often transported directly into the victim’s body. Puncture wounds can also be very difficult to properly clean. For these reasons, puncture wounds often take a long time to completely heal.

Wound Repair: When a cut extends through the epidermis and into the dermis, it almost inevitably causes damage to dermal blood vessels. Bleeding occurs at the site of the wound, which helps to clean the wound by flushing out bacteria and debris that might otherwise cause infection or impair healing. Damaged cells and mast cells in the area of injury release chemicals such as histamine that trigger an inflammatory response and also attract macrophages and fibroblasts to the injury site.

Inflammation occurs when nearby blood vessels become enlarged and more porous. This means that more blood is delivered to the site of the injury and that cells (e.g. macrocytes) and other substances can more easily move out of the blood and into the injury site. [For a small cut, inflammation increases blood flow to the injury site, but if the cut is large- and deep-enough that major blood vessels are affected, the reverse is true. Larger blood vessels constrict when they’re cut, which reduces blood flow to the injury site, making it less likely that you’ll bleed to death before the blood can clot.]

Increased flow of blood to inflamed tissues has several effects. First, it causes the injured area to become reddened, swollen, and painful. More to the point, the increased delivery of blood causes the damaged tissue to heat up. Since many bacteria and viruses can be killed by even a slight elevation of body temperature, this is an effective anti-infection measure. Second, increased delivery of oxygen and nutrients helps to support cells that are actively repairing damage and/or fighting off infectious agents. Third, increased blood flow to the injury site makes it easier for “white blood cells” (e.g. macrophages) and fibroblasts to reach it.

http://www.freethought-forum.com/images/anatomy7/wound_1.jpg
Healing of a Wound: Step 1
A freshly-made wound bleeds freely, which helps to flush bacteria
and other potential pathogens out of the wound.


Within a few minutes of the injury, the blood will have begun to clot (coagulate). When blood stops flowing it clots, largely because of the actions of thrombocytes and fibroblasts. Thrombocytes are normally restricted to the blood and don’t encounter collagen fibers. But when blood flows into an open wound, thrombocytes encounter exposed collagen fibers, to which they stick. When this happens, the thrombocytes are said to be “activated.” They release various chemicals that initate a clotting cascade. Among other things, this makes the thrombocytes themselves very “sticky,” and they clump together at the injury site. This clumping together of thromocytes wherever blood vessels are ruptured forms a platelet plug, which may be enough to seal the damaged blood vessels if the cut is a small one. Meanwhile, macrophages and fibroblasts enter the injury area. The fibroblasts release long, sticky strands of the protein fibrin. These strands act like a net to trap erythrocytes and other cells and hold them in place.

Within several hours of the injury, a scab will have formed. The scab consists of blood cells held together in a more or less solid mass by protein fibers secreted by fibroblasts and other cells. The scab seals the wound until more permanent repairs can be made. Fibroblasts also secrete collagen fibers that eventually knit the wound edges together and begin to pull the wound closed. Meanwhile, macrophages engulf and destroy bacteria, viruses, dead and damaged body cells, and other potential pathogens. Epidermal cells migrate down, along the edges of the cut and attempt to replace the missing epidermal cells.

http://www.freethought-forum.com/images/anatomy7/wound_2.jpg
Healing of a Wound: Step 2
A scab forms over the wound, preventing further blood loss. Macrophages enter the area and engulf viruses, bacteria, and dead or damaged body cells. Granulation tissue forms as blood capillaries grow into the clot, attracted by chemicals secreted by fibroblasts.


Dermal and epidermal cells replace the lost tissue as the fibrin clot begins to disintegrate. If the wound is a large one, however, neither the epidermal nor the dermal tissues can completely cover over the injury site. If the injury is too large to be repaired by epidermal and dermal cells, protein fibers laid down by fibroblasts fill the gap. In this case, the newly laid-down tissue will consist mostly of fibrous connective tissue. This replacement tissue is much denser than was the original skin, and is much less elastic. Relatively few blood vessels or nerves penetrate into it. Severely damaged hair follicles, sebaceous or sweat glands, muscle cells, or nerves cannot be repaired and are not replaced. This replacement tissue, typically lacking in hair, sweat or sebaceous glands, and with minimal muscles, nerves, or blood vessels, is known as scar tissue, and is apparently the body’s “best attempt” at repairing serious wounds.

http://www.freethought-forum.com/images/anatomy7/wound_3.jpg
Healing of a Wound: Step 3
As epithelial cells attempt to repair the wound, the scab begins to disintegrate. If the wound is too large for epithelial cells
to repair, scar tissue fills the gap. Scar tissue consists largely of fibrous connective tissue laid down by fibroblasts.

http://www.freethought-forum.com/images/anatomy7/wound_4.jpg
Healing of a Wound: Step 4
Scar tissue fills the gap of a large wound, leaving a scar that has few or no sweat glands, hair follicles, or sensory nerves.


Burns: There are three degrees of burns, depending upon the extent of damage they cause. First-degree and second-degree burns are called partial-thickness burns, because they affect only the superficial layers of the skin.

First-degree burns affect only the epidermis of the skin. Most sunburns, for example, are first-degree burns. The skin reddens and can become quite painful, but there is no permanent damage. This painful reddening of the skin is called erythema and is caused by inflammation of the affected tissues.

In second-degree burns, damage extends completely through the epidermis and perhaps into the dermis. Blistering, pain and swelling occurs, but accessory structures such as hair follicles usually remain intact. If the blisters rupture, they can easily become infected. These burns usually heal within 1 – 2 weeks, but some scar tissue may form because if the epidermis is destroyed, it cannot be replaced.

Third-degree burns are also known as full-thickness burns, because they penetrate all the way into the subcutaneous layer, destroying the epidermis and dermis in the process. Ironically, third-degree burns are less painful than are second-degree burns, because sensory nerve endings are destroyed, along with accessory structures, blood vessels, and other dermal components. Extensive third-degree burns cannot heal themselves, because epithelial and dermal cells are destoyed and cannot cover over the injury. This means that third-degree burns pose a serious infection risk.

Burns that cover more than 20% of the skin surface represent serious threats to survival. Extensive burns can cause disruption of the fluid and electrolyte balances. Even partial-thickness burns seriously degrade the skin’s ability to prevent water and electrolyte loss, and full-thickness burns can cause fluid loss through the skin to increase to five times the normal rate.

Increased loss of fluid across the skin surface means increased cooling of the skin from evaporation. This means that burn victims must expend more energy to keep their body temperatures up.

Burned skin surfaces, damp from fluid loss and covered in dead tissue, provide nearly ideal habitats for bacteria. Second- and third-degree burns readily become infected, and widespread bacterial infection, called sepsis (from the Greek, meaning “rotting”), is the leading cause of death in burn victims.

http://www.freethought-forum.com/images/anatomy7/burns.jpg
How to estimate the percentage of the body surface affected by a burn.


There are four crucial components to the treatment of full-thickness burns. The most immediate concern is to make sure that lost fluids and electrolytes are replaced. The next concern is to ensure that the victim is provided with sufficient nutrition to meet the increased metabolic demands for thermoregulation and for healing. Burn victims must be carefully protected from infection while they are recovering. The final component to treatment of full-thickness burns is repair of the damaged tissues.

Because full-thickness burns cannot heal on their own, surgical intervention is necessary if the victim is to have any hope of recovery. A skin graft can be used to transfer intact skin from another part of the body to cover the burn site.

Nowadays, a piece of healthy skin the size of a postage stamp can be removed from a person and cultured in a laboratory setting to produce a sheet of epidermal cells several square meters in area that can then be transplanted onto body surfaces. Skin grown this way typically isn’t as strong or as flexible as the original skin was, but it’s certainly a lot better than having no skin at all or being covered in scar tissue. With modern grafting techniques, a young victim with burns over 80% of his or her body has about a 50% chance of recovery. Just a few years ago, such a person would have had almost no hope of recovery.


[B]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.


http://www.freethought-forum.com/images/anatomy7/cyanosis.jpg
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.
http://www.freethought-forum.com/images/anatomy7/freckles00.jpg
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.

http://www.freethought-forum.com/images/anatomy7/vitiligo01.jpg
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: http://www.freethought-forum.com/images/anatomy7/vitamin_d.gif
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.)

http://www.freethought-forum.com/images/anatomy7/sunbathers.jpg
These women are diligently working to build strong bones and teeth.
They’re also hoping to reduce their risk of developing ovarian cancer.


[B]Integumentary System Disorders:

Disorders Due to Trauma: A localized shedding of epithelial tissue is known as an ulcer. If blood flow in the dermis is interrupted, epidermal cells may die from lack of nutrients. Pressure on the skin can force blood out of the dermal blood vessels, as you can easily see for yourself if you have fair skin. Press down hard on the back of your hand, where the skin is fairly thin, then quickly release the pressure. You’ll be able to see that the skin has gone white from blood being forced out of it, then it reddens as blood flows back into it.

The same sort of thing happens if you lie in bed in the same position for long enough. Where there’s sufficient pressure on the skin, blood will be forced out, and if this happens for a long-enough time, epidermal cells in the affected area will begin to die and slough off. This is what causes decubitis ulcers (bedsores) in bedridden persons who aren’t moved often enough or given massages to stimulate cutaneous blood flow.

Disorders Due to Infection: Most teens or ex-teens are familiar with acne vulgaris. This occurs when the ducts of sebaceous glands become blocked and inflamed. Bacterial infection causes accumulation of pus, and a pimple results. The sebaceous glands become more active when testosterone levels rise and thus more likely to become clogged by their secretions, which is why acne tends to afflict people when they reach puberty. Incidentally, there’s no convincing evidence that eating chocolate, french fries, or sugar in any way contributes to acne. People are variable, however, and your diet certainly affects your overall health; with that in mind, it’s possible (but by no means demonstrated) that certain foods make sensitive individuals more prone to developing acne.

Folliculitis is inflammation of hair follicles. If hair follicles are damaged by friction from clothing, shaving, or other causes, they may then become infected with bacteria. A serious infection can cause a large amount of pus to accumulate, and the result is a furuncle or boil.

Inflammation of the skin is known as dermatitis. Inflammation accompanied by edema (swelling), itching, dryness, and flaking of the skin is known as eczema. Forms of dermatitis include seborrhoeic dermatitis, which occurs when the yeast Malassezia furfur infects the skin. In most people, a yeast infection is harmless, but a severe infection causes inflammation of the skin and shedding of epidermal layers.

Contact dermatitis occurs when some sort of irritating chemical causes the skin to become inflamed. Poison ivy (Toxicodendron radicans), for example, produces a chemical known as urushiol, which causes severe itching and inflammation when it contacts the skin of a sensitive individual.

Infection of the scalp by Malassezia furfur can cause dandruff. Apparently in an attempt to cope with the infection, the body increases the rate of production of epidermal cells in an infected region. (Presumably, this is an adaptive response to try to slough off the affected tissue and get rid of the fungal infection in the process.) The result is that skin cells are shed rapidly and in large clumps.

While fungal infection is the most common cause of dandruff, it can also be caused by an usually dry scalp or by overactive sebaceous glands. For some reason, zinc deficiency seems to make one more prone to develop dandruff.

Skin Cancers: Exposure to ultraviolet radiation in sunlight can damage DNA in the nuclei of skin cells, causing them to become cancerous. Basal cell carcinoma occurs when cells in the stratum basale become cancerous. (Carcinoma, as you recall, is any cancer of epithelial tissue.) While it can be disfiguring, basal cell carcinoma is not generally considered life-threatening. Another common type of skin cancer is squamous cell carcinoma. Exposure to sunlight can cause it in the skin, where it usually isn’t life-threatening, but squamous cell carcinoma can affect other parts of the body where squamous epithelium is found, including the esophagus, the cervix, and the lungs. Smoking is thought to be a major trigger for these more threatening carcinomas.

Melanoma occurs when melanocytes become cancerous, and it is a life-threatening condition. Any time you exposure yourself to enough UV radiation to develop a sunburn, you’re increasing your likelihood of developing melanoma. The “ABCDE” mnemonic (Asymmetry, Border, Colorful, Diameter, Evolution) may be of help if you’re worried about the possibility of developing melanoma.

An Asymmetrical skin lesion that has no obvious cause such as an injury is cause for concern, particularly if it doesn’t appear to be healing. If the Border of the lesion is irregular, that is a cause for concern. Melanomas are usually Colorful. “Moles” that are greater than 5 millimeters in Diameter are suspicious. If a mole or lesion changes in size or shape (Evolves), that is a cause for concern.

Inherited Disorders:
Erythropoietic porphyria is an inherited condition that makes sufferers’ skins extremely sensitive to sunlight. Afflicted individuals may develop dermatitis from even a brief exposure to sunlight, and more prolonged exposure will cause blistering of the skin. Long-term exposure will cause severe pain and even death of skin tissues. There’s some suspicion that people suffering from erythropoietic porphyria helped to inspire the vampire legends.

Epidermolysis bullosa simplex is an inherited condition in which the victim cannot manufacture normal keratin. As a result, the skin is so fragile that even the slightest physical contact can cause the skin to blister or even cause the epidermis to peel away from the dermis. Severely afflicted individuals develop scar tissue over much of their body surfaces.

Ichthyosis congenita (harlequin-type ichthyosis) is a particularly horrific condition that is more or less exactly the opposite of epidermolysis bullosa simplex. In ichthyosis congenita sufferers, the epidermis produces so much keratin that the skin hardens into massive, diamond-shaped “scales” or “plates.” (The name “ichthyosis” refers to the skin looking like a fish’s scales.) Children born with this condition rarely live for more than a few days, as they’re covered in what is essentially armor plating and their movements are severely restricted. Where the skin of an unafflicted person would bend, thiers cracks, leaving them at great risk of infection. With modern treatment techniques and constant care, some “harlequin babies” have managed to survive into adolescence.

Aging and the Integumentary System: Exposure to sunlight over a person’s lifetime causes connective tissue fibers in the skin to gradually lose their elasticity, which makes the skin stiffer and more leathery. Ultimately, those who spend lots of time in the sun will often find that their skin begins to sag and wrinkle as elastin fibers are damaged by solar radiation.

Some people have collagen injected directly into the skin to treat wrinkles caused by sun damage. This can indeed temporarily smooth wrinkled skin, but the collagen fibers don’t become incorporated into the skin tissues, so it’s only a temporary measure. Some people try smearing collagen-containing creams on their skin to achieve the same effect, but they’re just wasting their time and money; collagen is much too large a molecule to be absorbed through the skin surface.

Aside from sunlight-induced damage, the skin changes in many ways as we age. For one thing, the activity of cells in the stratum basale decreases as we age, so the epidermis grows more slowly and becomes thinner. This means that older people are more susceptible to injury and skin infections.

The number of immune-system cells (Langerhans cells) in the skin decreases as we age. This, too, makes us more vulnerable to skin infections.

Vitamin D production decreases as we age. This can cause the muscles and bones of older people to become weaker.

Melanocyte activity decreases as we age, which makes the skin become paler. Especially among caucasians, this makes older people more susceptible to sunburn.

Glandular activity decreases as we age. Decreased activity of sebaceous glands can cause drying of the skin, and decreased sudoriferous gland activity means that older people cannot shed body heat as rapidly as can younger people, which makes them vulnerable to overheating in warm environments.

The blood supply to the dermis is reduced, even as sweat gland activity decreases. Because of the reduced blood flow, the skin feels cooler, which can trigger thermoreceptors (temperature-sensitive nerve endings) in the skin, making the person feel cold, even in a warm room. But because of the reduced dermal circulation and reduced sweat gland activity, overexertion in an effort to warm up can cause dangerous overheating of the body.

Hair follicles stop functioning entirely or produce thinner hairs as we age, so the hair thins. As melanocyte activity decreases, the hairs turn to gray or even to white.

Development of the Integumentary System: Humans, like virtually all animals, are triploblasts. What this means is that, very early in fetal development (just a few days after we’re conceived), we consist of three layers of tissues. The outermost layer of tissue is known as the ectoderm, the innermost layer is known as the endoderm, and the middle layer is known as the mesoderm.

Most of the digestive system develops from endodermal tissue, but the skin doesn’t, so we won’t consider that for now. Most of the muscles of the body develop from mesoderm; as you might imagine, this includes some of the dermal tissues. However, most integumentary tissues – including the epidermis, hair follicles, sweat glands, and sebaceous glands – develop from ectodermal tissue.

Some Concluding Thoughts: The integumentary system forms the first line of defense against the outside world. It protects us against dehydration, abrasions, cuts, ultraviolet radiation, noxious chemicals, infectious agents, and a whole host of things that would otherwise quickly kill us. That’s a pretty big job for a membrane that’s much less than an inch thick. All in all, it does a pretty darned good job, I’d say.


If the integumentary system is the one that we present to the world, the one that makes us who we are by shaping the body itself is the Skeletal System. We’ll consider that one next.