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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
01-07-2007
Default Sebaceous Glands

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.


Why 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.


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.


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Ensign Steve (12-13-2008)
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