Explaining the integument’s structure

Touching the epidermis

The most familiar aspect of the integument is the epidermis; it’s the part you see when you look in the mirror or at others. The epidermis feels soft, slightly oily, elastic, resilient, and strong. In some places, the surface contains dense, coarse hairs; in other spots, it has a lighter covering of finer hairs; and in a few places, it has no hairs at all. The nails cover the tips of the fingers and toes.

The epidermis is composed of stratified squamous tissue and has no direct blood supply. Its nourishment diffuses through the basement membrane from the dermis below. As your skin cells age and get pushed further away from the resource supply, they weaken and eventually die. This gives the epidermis its layered appearance (not visible to the naked eye though). Figure 4-1 illustrates the layers of the epidermis.

The healthiest, happiest layer of epidermal cells is along the basement membrane, called the stratum basale (or stratum germinativum). This layer is the only one that has cells that reproduce. The older cells shift upward, becoming spindle-shaped, forming the stratum spinosum, the thickest of the epidermal layers. Next is the stratum granulosum; cells in this layer are flattened and the nuclei and organelles start to shrivel. The most superficial layer is the stratum corneum; these are hardened, dead cells full of keratin. Thick skin, such as the palm of your hands, has an additional layer between the granulosum and corneum called the stratum lucidum; named because the cells appear lucid (or clear) under a microscope, this layer is built up for added protection in areas receiving a lot of wear and tear.

Cells are shed continuously from the top of the epidermis and replaced continuously by cells that are pushed up from the deeper layers. The entire epidermis is replaced approximately every six to eight weeks throughout life.

 

The thin, impervious cover

Think of the stratum corneum as a sheet of self-repairing fiberglass over the other layers of the epidermis. It’s only 25 to 30 cells thick, dense, and relatively hard. All the cells are keratinized, as they became filled with this protein (keratin) while they are pushed toward the surface. The keratin and other proteins mechanically stabilize the cell against physical stress. Sebaceous glands in the dermis release sebum onto the surface of the stratum corneum for softening and waterproofing. The stratum corneum protects the entire body by making sure that some things stay in and everything else stays out. (See the nearby sidebar "Cosmetics and the stratum corneum".)

One important thing that the stratum corneum doesn’t seal out, however, is ultraviolet radiation. This form of energy goes right through the skin’s surface and down to the layers below, where it stimulates the production of vitamin D. In high doses, it burns the skin and damages DNA, which can cause cells to become cancerous. Some exposure to UV radiation is necessary for our health, but too much UV radiation can clearly be problematic. Special cells in the stratum basale produce melanin, which absorbs harmful UV radiation and transforms the energy into harmless heat. More on this later in this section.

 

Cosmetics and the stratum corneum

The use of cosmetics is far older than recorded history, along with other appearance-enhancers and subgroup identifiers like tattoos and scarifications (burning or etching). Two types of cosmetics commonly used in culture today are moisturizers and exfoliants.

 

Moisturizers

What do moisturizers do? After the water in them evaporates into the air, the lipids they contain remain on the skin’s surface, adding another waterproof barrier to prevent the escape of water from the skin. This increases the very scant water in the layer and helps block particles and chemicals from getting through. That’s it.

As far as their effects on the skin, all moisturizers are essentially the same, whether they’re petroleum jelly, a luxury-brand "serum", a drugstore lotion, or a vitamin E cream from the health food store. Not that they don’t contain the ingredients the manufacturers claim; it’s just that none of them gets past the stratus corneum. All those age-defying peptides, metallic microparticles, organic extracts, vitamins, rare botanicals, and antioxidants remain within the lipid layer on the skin’s surface and go down the drain with the lipid and the trapped dirt when the skin is washed.

 

Exfoliants

Speaking of washing, what about exfoliants? Exfoliation means to remove the topmost cells of the stratum corneum by mechanical or chemical means. Your skin sheds these dead cells all the time, but exfoliation hurries up the process for those cells it reaches. Generally, exfoliation neither hurts nor helps any physiological process in the skin: The stratum corneum remains an effective barrier. Some people think that having some slightly younger (but still dead) cells on the skin’s surface enhances its appearance.

Shaving is one common form of mechanical exfoliation as cells are removed as the hairs are cut. A mechanical exfoliant cosmetic (such as sugar scrubs) usually contains an abrasive substance embedded in the soap. Different people, and different skins, prefer different abrasives. Chemical exfoliation (such as masks and peels) usually involves the application of an acidic substance that breaks up the fibers holding the cells of the stratum corneum together. But no worries — ranks of keratinized cells are always moving up from below to take their place. The chemicals may also produce a slight irritation that leads to a temporary inflammation reaction, puffing out some shallow wrinkles in the skin (again, the goal being attractiveness).

Does that mean moisturizers and exfoliants are a waste of money? Not necessarily. Keeping the skin moist, lubricated, and clean feels good and is a good thing generally. You may choose one cosmetic over another for a number of reasons: nicer texture, nicer perfume, the absence of perfume or ingredients that irritate the skin. And for some people, the use of an expensive self-care product enhances self-esteem ("Because you’re worth it", as the advertising tag line of one brand says). Self-esteem and self-confidence enhance psychological well-being, which may enhance reproductive success and survival.

 

The transit zone

The stratum lucidum, found only on the palms of hands and soles of the feet (thick skin); the stratum granulosum; and the stratum spinosum lie in distinct layers below the stratum corneum. Old cells slough off above and new cells push up from below, finally getting up into the stratum corneum. The process takes about 14 to 30 days. Most of the cells that comprise the epidermis are called keratinocytes. These cells create structural proteins (like keratin), lipids, and even some antimicrobial molecules. As they are pushed away from the nutrient source (blood vessels in the dermis) they become progressively more keratinized. These layers also contain Langerhans cells, immune cells that arrest microbial invaders and transport them to the lymph nodes for destruction.

 

Caring about keratins

Keratins are fibrous proteins produced by skin cells in mammals, birds, and reptiles. Keratin-containing anatomical structures are hard, tough, and waterproof. The α-keratins, are the main components of hair — including wool, nails, claws, hooves, and horns. (But not antlers, they’re derived from bone.) The baleen of filter-feeding whales is made mainly of α-keratins. The other type, the β-keratins, are even harder and are found in human nails, mammalian claws, and porcupine quills; in the shells, scales, and claws of reptiles; and in the feathers, beaks, and claws of birds. People who make educated guesses about the anatomy and physiology of dinosaurs believe that keratins were likely a major component of the claws, horns, and armor plates of these creatures.

 

The cell farm

The stratum basale, also called the stratum germinativum or basal layer, is like a cell farm, constantly producing new cells and pushing them up into the layer above. This layer also contains melanocytes, which produce the melanin pigment that gives color to your skin, hair, and eyes and protects the skin from the damaging effects of UV radiation in sunlight. Melanin absorbs UV radiation and dissipates more than 99.9 percent of it as heat.

Everybody’s stratum basale has about the same number of melanocytes (half a million to a million or more per square inch of skin), but the amount of melanin produced varies depending mainly on genetics (heredity). The environment can also play a role in variety of skin color because exposure to UV radiation stimulates increased production of melanin. Human groups living close to the equator have evolved genes that stimulate melanocytes to produce more melanin as protection from UV radiation. Without the melanin, the radiation can burn the skin, damage DNA, and ultimately cause skin cancer.

 

The rise and fall of a tan

Melanogenesis, the production of melanin in response to UV radiation, is the physiological term for tanning. The more exposure skin has to UV, the greater the production of melanin. The melanin is released from the melanocytes and absorbed by skin cells that journey to the stratum corneum and are sloughed off. So your tan falls off, cell by cell, but the DNA damage to the cells of the stratum basale remains for your lifetime.

 

Exploring the dermis

Below the layers of the epidermis and several times thicker is the dermis. The dermis itself is made up of two layers: the papillary region and the reticular region.

 

Under the basement

The papillary region is made up of the basement membrane, which sits just below the epidermis, and papillae (finger-like projections) that push into the basement membrane, increasing the area of contact between the dermis and the epidermis. In your palms, fingers, soles, and toes, the papillae projecting into the epidermis form friction ridges. (They help your hand or foot to grasp by increasing friction.) The pattern of the friction ridges on a finger is called a fingerprint.

The papillary region is an example of a common anatomical "strategy" for increasing the surface area between two structures. More areas of direct contact lead to more chances for molecules to travel from one side to the other. Think about the difference between leaving a crowded parking lot after a concert with only 2 exits and one with 20 exits. We discuss another prominent example, the intestines, in Chapter 11.

 

A manufacturing site

The reticular region is chock-full of protein fibers and is a complex and metabolically active layer. Cells and structures of the reticular region manufacture many of the skin’s characteristic products: hair and nails, sebum, watery sweat, apocrine sweat. (See the "Accessorizing Your Skin" section later in the chapter.) The region also contains structures that connect the integument to other organ systems: sensory structures to communicate with the nervous system, lymphatic vessels, and a very rich blood supply.

The blood vessels in the dermis provide nourishment and waste removal from its own cells as well as from the stratum basale. The blood vessels in the dermis dilate (become larger) when the body needs to lose heat and constrict to keep heat in. They also dilate and constrict in response to your emotional state, brightening or darkening skin color, thereby functioning as social signaling.

 

Getting under your skin: the hypodermis

The subcutaneous layer (or hypodermis, or superficial fascia) is the layer of tissue directly underneath the dermis. It is mainly composed of connective and adipose tissue (fatty tissue). Its physiological functions include insulation, the storage of energy, and help in the anchoring of the skin. It contains larger blood vessels, lymphatic vessels, and nerve fibers than those found in the dermis. Its loosely arranged elastin fibers anchor the hypodermis to the muscle below.

The thickness of the subcutaneous layer is determined in some places by the amount of fat deposited into the cells of the adipose tissue, which makes up the majority of the subcutaneous layer. Recently, researchers have found that adipose tissue also plays a very active role in the endocrine system (see Chapter 8).