You may be surprised to learn that your skin is the largest organ of your body, measuring approximately 20 square feet and weighing, on average, between 7 and 9 pounds. Normal, healthy skin is a complex organ composed of many layers. At its thickest (back, soles of feet, and palms), your skin measures approximately ‘/8 inch. At its thinnest (eyelids), it measures only ½5 inch. Among its many important functions, skin not only protects the inside of your body from the assaults of the outside environment, but it also serves as the primary regulator of body temperature.
Skin is divided into three layers: the epidermis, dermis, and subcutis. Each of these layers has its own unique functions. The epidermis is the highly cellular uppermost layer of your skin. It is no thicker than a page in this book and is composed of fifteen to twenty layers that overlap near the skin surface. As you can see, the epidermis is itself subdivided into three layers. The bottommost layer is appropriately called the basal layer, because the cells composing it form the base of the epidermis. The basal layer may also be called the Stratum germinativum because the cells in this layer continually germinate (“give birth to”) new cells.
The layers of the epidermis undergo a continual process of birth, life, and death. As older cells are shed at the skin surface, new cells are formed in the basal layer. These newly formed cells grow, mature, and divide to produce more cells. Finally, they make a two-week migration upward through the epidermis to the surface of your skin, replacing older cells being shed there.
The basal layer is not composed exclusively of basal cells. It hosts another important class of cells called melanocytes, the pigment-producing cells of your skin. They produce the melanin, which is responsible for imparting a color to your skin. Usually, every sixth cell in the basal layer is a melanocyte.
Racial differences in skin color are attributable to genetically determined differences in the amount and distribution of melanin. As a rule, people with darker skin possess more melanin in their epidermal cells than people with fair skin. In addition, the melanin within the skin cells of dark-complected people is more densely arranged; by contrast, melanin is more sparsely arranged in people with fair skin.
Sunlight stimulates melanin production and a suntan is nothing more than sun-induced melanin production. More than simply imparting color to your skin, melanin protects you by absorbing the sun’s damaging ultraviolet rays. Unfortunately, this protection is far from complete; long-term sun exposure can result in premature aging of the skin and the development of skin cancers.
The top layer of the epidermis consists of a sheet of nonliving cells called the Stratum corneum, or the horny layer (which gets its name from the fact that when tightly compacted, its cells become tough, like the horn of an animal. In fact, the horns of mammals are made of the same protein material, keratin, which makes up the horny layer of human skin). The surface of the horny layer is somewhat acidic and is referred to as the acid mantle.
Horny layer cells are constantly shed at the skin surface, remaining there about two weeks and replaced from below. Any abnormal accumulation of horny cells on the surface of your skin, for any reason, can result in skin ashiness or flakiness. Skin ash is particularly troublesome to people with dark skin because of the sharp contrast in color between the gray ash and the surrounding skin.
The horny layer of your skin serves several extremely important functions. It is not only the major physical barrier to the environment but, to sonic extent, is also a shield against the sun’s harmful ultraviolet rays. More importantly, it effectively prevents the penetration of most environmental substances that come in contact with your skin or are applied to it. In general, only those substances possessing a molecular size smaller than the size of water molecules can readily penetrate this epidermal barrier. This is the main reason why, contrary to what cosmetic manufacturers would have you believe, your skin cannot “eat up” or “drink up” such, substances as collagen, elastin, vitamins, or nutrients—ingredients that are often contained in many of the fanciest, most expensive moisturizers and cosmetics. The molecular structure of these ingredients is simply too large to pass through your skin.
The middle and thickest, layer of the epidermis is called the Stratum spinosum or prickle cell layer, owing to the spiny, hair-like, prickly-looking processes, or projections, that link the cells in this layer. The cells within the prickle cell layer are known as squamous cells. Essentially, squamous cells are basal cells that have matured and migrated upward within the epidermis.
Beneath the epidermis lies the dermis, the cellular and largely fibrous and elastic supporting layer of your skin. It contains the important fibers collagen and elastin, which are the subject of much advertising hype by moisturizer manufacturers. Collagen and elastin are the complex proteins responsible for the support and elasticity of the skin. They enable your skin to regain its shape after being stretched or pulled. Both proteins are composed of large, complex molecules, far too large to be “eaten up” or “drunk in” by your skin (thereupon to “replace lost natural protein”), as some ads suggest.
The dermis is home to other important skin structures as well. The tiny, twig-like sensory nerve endings that allow you to sense, for example, something as soft as a wisp of cotton on your skin or subtle changes of temperature are located high in the dermis, as are the nerve endings that allow you to feel pressure, vibration, and pain.
Skin nutrition and oxygenation are supplied by the numerous tiny arteries, veins, and capillaries coursing upward through the dermis. These vessels branch from larger vessels situated more deeply in the body. Incredibly, each square inch of the dermis houses 15 feet of small, nutrient-providing blood vessels. Their constriction and dilation, in response to extremes of heat and cold, are responsible for keeping your body temperature constant. These small blood vessels also keep your skin healthy and viable and remove metabolic waste materials. As a rule, nutrients cannot be supplied to your skin by topical application. In other words, applying vitamins, minerals, fruits, or vegetables, or any cream or lotion concoction, to your skin in hopes of getting these additives into your skin is a waste of your time and money.
The subcutis, or fatty layer, lies below the dermis and is the bottommost layer of your skin. The fatty layer functions both as a cushion for your vital internal organs and a reserve energy storage site for the body. The amount and distribution of fat throughout the body is believed, in most cases, to be an inherited family trait.
SPECIAL SKIN STRUCTURES
The skin houses several other important structures. These include hairs, hair follicles, and three kinds of sweat glands. Hairs, which are nonliving, fibrous strands, are, like the horny layer of the skin, composed primarily of the protein keratin. Hair follicles, located at the base of the hair shaft, generally in the deep dermis or subcutis, are the living reproduction parts of the hair.
The skin contains three types of sweat glands—the sebaceous, apocrine, and eccrine glands. The sebaceous glands, or oil glands, lie to the side of the hair follicles. Sebaceous glands produce se/mm, or natural skin oil, which is actually a complex mixture of a number of different fats and waxes. The oil you feel on your face on a hot, humid day is sebum. Approximately three thousand oil glands are contained in each square inch of the forehead and facial skin.
Sebaceous glands are located primarily on the scalp, face, chest, and back, although they may be found elsewhere on the body. Present from birth, these glands mature and begin to secrete sebum actively following puberty. Sebum is secreted through a small duct leading directly from the oil gland into the hair shaft, and travels upward to the skin surface through the hair follicle. Dead cells and other debris within the hair follicles are also “washed” to the skin surface by this process. Sebum coats your skin, locking in its natural moisture and preventing it from drying out.
Apocrine glands are located primarily in the armpit, genital and anal areas, and around the belly button. These glands are heavily coiled and are usually situated deep within the subcutis. They produce a milky sweat that when broken down by bacteria on the skin causes body odor. Apocrine glands secrete their contents into the upper portion of the hair follicle, and from there the secretions exit to the skin surface.
In other mammals it is believed that apocrine glands produce body odors that serve as a sexual function to attract mates. In humans, however, the function of these glands remains unknown. They are present in the skin from birth, but do not mature and secrete until the onset of puberty. Periods of heightened emotional stress appear to stimulate their secretions.
Like apocrine glands, eccrine glands are generally situated in the subcutis, and are heavily coiled. Unlike sebaceous and apocrine gland secretions, however, eccrine sweat exits the skin through its own pores rather than through the hair follicles. Exercise, hot weather, fever, and emotional stress are known to stimulate eccrine sweating. Your skin contains between 2 and 3 million eccrine sweat glands; they are located over the entire body, but are highly concentrated on the palms, soles of the feet, and armpits. In these areas, and on the forehead, sweat production appears to be more strongly linked to emotional and stress factors than to heat stimulation. Since eccrine sweat is largely water, these glands are normally not responsible for body odor.
The watery, colorless sweat produced by eccrine glands functions to regulate body temperature. Sweat evaporation at the skin surface lowers body temperature. In addition, sweat can help to eliminate small amounts of waste salts and other substances from the body. However, some people suffer from a profuse sweating condition known as hyperhidrosis. This condition, which often requires medical treatment, can pose a severely compromising and embarrassing social problem for sufferers.
Finally, a word about the all-important subject of pores. A pore is the lay term for the opening of your oil or sweat glands at the skin surface. (The medical term is follicular orifice.) The size of your pores is largely determined by heredity. Unfortunately, although there are a number of toners, astringents, and foundations that claim to be able to shrink dilated pores, no product is yet available that can actually accomplish this for more than a few hours. If you have acne or oily skin, which can make your pores appear wider, consult your dermatologist.
Excerpted from Super Skin--A Leading Dermatologist's Guide to the Latest Breakthroughs in Skin Care, by Nelson Lee Novick, M.D.
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