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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 Skin and the Integumentary System

Skin and the Integumentary System:

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.

The cutaneous membrane and the hypodermis.
The hypodermis (subcutaneous layer) is not actually part of the skin.

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.

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:
Lines 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.


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Ensign Steve (12-13-2008)

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