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Intro to Anatomy 7: The Integumentary System
Intro to Anatomy 7: The Integumentary System
The Lone Ranger
Published by The Lone Ranger
Default Accessory Organs of the Skin

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.
Hair Follicles and Hair:

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:

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.

“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.


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Ensign Steve (12-13-2008)

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