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What Covers Us?
The finest clothing made is a person's own skin, but, of course, society demands something more than this.
Our skin, at the most basic level, defines us.
The skin is the body's largest organ, averaging twenty square feet and nine pounds; it makes up 16 percent of the body's weight. The skin is complicated but amazing in structure; it can be the target and dwelling place of thousands of tiny viruses, bacteria, or yeasts, yet does a stellar job of living with the good and keeping out the bad, all to protect the inner environment.
Epidermis, Dermis, Subcutis: A Tour of the Skin
The epidermis, the outermost layer of skin, is a major part of the immune system. It is filled with Langerhans cells that form a first line of defense against environmental threats, identifying foreign materials and dangerous substances and ridding us of them. The epidermis is also part chemical plant, synthesizing vitamin D in the presence of sunlight, transforming a variety of helpful chemical compounds that interact with it, and inactivating substances that could be dangerous for us to absorb.
The epidermis protects the body from all kinds of insults. As Arthur and Loretta Balin write, "It seems fitting that the organ that defines the boundary between outside and inside worlds would be involved in the essential immune-system task of distinguishing between self and other." Poison ivy is a common example: it is known to effect over 350,000 people in the United States annually, and demonstrates the wide range of possible sensitivities and reactions to exposure. Here's how the reaction to poison ivy works. Urushiol is a chemical within the sap of the poison ivy plant that binds to the skin on contact. Urushiol shimmies its way into the skin and is broken down by called T lymphocytes (or T-cells) that recognize it as a foreign substance, or antigen. The T-cells send out inflammatory signals called cytokines and the immune system pumps up the volume and calls in the troops-white blood cells. Still under the command of cytokines, the white blood cells turn into macrophages (super Pac Men) that eat up the foreign urushiol but also cause collateral damage to the normal tissue skin, resulting in inflammation and a dermatitis. A severe allergic reaction and blistering and oozing may occur in susceptible individuals; the fluid is produced by the body as blood vessels develop gaps and leak fluid through the skin. Approximately a quarter of the people exposed to poison ivy have no allergic reaction, while in extreme cases a anaphylactic reaction can occur. With age and repeated exposure, the sensitivity usually decreases.
The epidermis has a top layer called the stratum corneum, composed of closely-packed cells that protect the skin from external abuse. The stratum corneum keeps the skin hydrated, both absorbing water and preventing water evaporation by means of a dense network of the protein keratin. The stratum corneum's thickness varies throughout the body. On the palms of the hands and the soles of the feet, this layer is thicker to provide additional protection. Generally, the stratum corneum has fifteen to twenty layers of dead cells in addition to the layers of proteins, for a total thickness of between ten and forty micrometers-very thin.
Every move you make results in showers of skin particles from the epidermis released into the air. Every twenty-four hours an estimated ten thousand million skin scales or squames peel off each of our bodies, accounting for one to one and a half grams of skin each day, or about one pound each year. These scales are the desiccated remnants of skin cells that continually form at the base of the epidermis and travel slowly toward the surface. After forty to fifty-six days a newly formed cell reaches the surface and it is now called the stratum corneum, the same horny components that make up our hair and finger-nails.
At high magnification this surface of dead skin appears as irregular patches of rough and curly cornflakes. House dust consists of 80-90 percent skin; squames are the motes in the sunbeam filtering into our rooms.
Just beneath the stratum corneum live the keratinocytes or squamous cells, which mature and move toward the surface to form the stratum corneum. Below them, in the deepest layer of the epidermis, is the basal layer, containing cells that continually divide and form new keratinocytes, replacing the old ones shed from the skin's surface. This constant upward migration characterizes the epidermis.
Also in the basal layer are the cells known as melanocytes, which determine differences in skin color (more about this in chapter 4). These cells produce the pigment melanin, which protects the skin from sunlight and determines the intensity of skin and hair color. Each of us has the same number of melanocytes. The difference between darker and lighter skin tones is a result of the type, amount, and arrangement of the melanin produced by our melanocytes. Those with darker skin color, such as African Americans, have more melanin and are much better adapted to the harsh conditions of sun exposure. Carotenes, mostly located a level deeper down in the dermis, provide the yellowish cast characteristic of Asian skins. Hemoglobin, the oxygen-carrying pigment in blood, gives pinkness to some fair skins. Freckles are due to increased melanin production, while nevocellular nevi, or moles, are caused by tightly packed groups of melanocytes. Solar lentigines, the flat, brown "liver spots," occur because of an abnormal increase in the number of melanocytes. Those with vitiligo have a decrease in melanocyte function and albinos are genetically unable to produce any melanin at all.
But the epidermis does not only protect and color us. It also is host to some of the marks of aging and sun exposure. If you look closely at the skin of the elderly, you may notice the acquired spots and "barnacles of life" (seborrheic keratosis) that accumulate in the epidermis. Scaly actinic, or solar, keratosis can be detected by feeling its sandpaper texture.
Below the epidermis is the dermis, the middle layer of the skin. It contains blood vessels, lymph vessels, sweat glands, collagen bundles, fibroblasts, and nerves. Held together by a protein called collagen made by the fibroblasts, the dermis is a major contributor to the skin's flexibility and strength and also contains pain and touch receptors.
The dermis also contains the hair follicles; infection or inflammation associated with infection in the vicinity of the roots results in folliculitis. As the Balins have pointed out, we humans call ourselves "naked apes," yet we are covered with fine, unpigmented hairs that are actually ultrasensitive touch sensors. As the only mammals with such highly sensitive touch receptors all over our bodies, we require an enormous brain to process this constant sensory input from the skin.
The dermis can be marked by dilated blood vessels called telangiectasias, or spider veins, brought on by chronic sun exposure, by estrogen, or in some cases by an underlying liver or blood disorder. (As we will see in part 2, every skin disorder has its anatomical correlate; the field of dermatopathology specializes in connecting clinical skin findings to underlying anatomy.)
The subcutis (also known as the subcutaneous layer) is the deepest layer of skin. It consists of a network of collagen and fat cells. This layer helps conserve the body's heat and protects the body from injury by acting as a "shock absorber."
When we are babies, our skin is elastic and resilient-and it becomes less so every day from then on. We lose about 1 percent of our collagen, as well as elastic fibers and blood vessels that attach to the epidermis, every year after age thirty. The result is crinkles and wrinkles, a rather unfair exchange. The skin becomes sallow and pale. We increase fat deposition in the areas we don't like, and we lose fat and therefore insulation in other body areas such as the face, arms, and legs. The underlying tissue depletion makes us more prone to injury, and the loss of nerves decreases our tolerance for cold.
Diagnosis and Ecological Detective Work
In examining patients who have skin problems, we note the morphology of individual lesions, their pattern in relation to each other, and their distribution on the body. Since the earliest days of medicine physicians have been observing skin diseases and classifying them by these three criteria. Skin diseases are generally dynamic processes that evolve over their course. Dermatologists often find it helpful to identify "primary lesions," which are the earliest abnormalities, and "secondary lesions," into which they may evolve. Understanding this evolutionary process makes understanding the pathophysiology of the disease possible.
Skin diseases are generally categorized as tumors (abnormal masses of tissue that may be solid or fluid-filled and can be benign, pre-malignant, or malignant, that is, cancerous), pigmentation abnormalities (such as birthmarks, melasma, vitiligo, and other pigment disorders), papulosquamous diseases, vesiculobullous diseases, papular eruptions, eczematous dermatitis, hypersensitivity reactions, cutaneous infections and infestations, and diseases of the skin appendages (hair, nails, glands, blood vessels).
I greatly respect other medical providers and their tools of diagnosis for inner maladies, from the medical imaging of radiologists to the scans and tests of neurologists. A pathologist can be an enormous aid to a dermatological diagnoses, adding another set of eyes and a deeper view of a disease process caught in a moment in time. Yet perhaps no other field of medicine entertains the notion of visual, real-life pattern recognition more than dermatology. Dermatology lives in the observable and the palpable-the skin. Skin clinicians deal with life in the wild, not tamed and frozen tissue samples from removed body parts and not representations on a screen.
I have always tended to look at the skin from the perspective of habitat and ecology. When I am not seeing patients in my office, I am often in the natural environment of Florida, hiking, kayaking, taking photos, and looking around. I strive to evaluate a skin lesion in the same way. If I see a series of red, dry, scaly actinic keratosis (AKs)-the so-called pre-cancers-I look for the more fully developed cancers. AKs live along the continuum from early stages to more raised and hypertrophic lesions to fully robust creations that manifest as squamous cell carcinomas (SCCs). If I can rid a patient's skin of AKs at an early stage, I eliminate the invasive species before it brings on more damage.
The natural habitat of these AKs and SCCs is on the sun-exposed areas of the skin, particularly on the left arm of those who keep an arm out the window while driving, the face, the neck, the chest, the legs, and any other areas of chronic sun exposure. In similar fashion, one is most likely to see a cypress tree with its stabilizing knees around the edge of a river or a ring-necked snake carrying its orange belly over moist woodlands.
All of this comes into perspective when discussing these maladies with my patients. Dermatology skills include the practical evaluation of the topography and climate of the skin. Who hangs out with this particular skin character? What will most likely provide benefit or do any harm? The more deeply I know the disease, the more deeply I can understand the prognosis and potential treatment. Like an ecologist, I have to know where to look if I want to find a familiar species: basal cell cancers (BCCs) and SCCs live in the more superficial skin layers, while melanoma invades subcutaneously. Sweaty armpits (for medical students and fans of Latin, intertriginous areas of the axillae) and upper thighs are delightful arenas for fungi to frolic and breed. The larger environment matters too: to give just one example, the rampant tinea versicolor has been reported to infect up to 20 percent of the population of Florida at any one time.
What would you expect to find as you travel over any particular person's cutaneous terrain? If I encounter the short timber of a cutaneous horn-a hard, horn-shaped tumor-I know to be cautious and to collect a sample that includes the base of the tumor when doing a biopsy, where the squamous cell layer may harbor a cancer. If a patient comes to me with a fire-like eruption on the face, neck, or other areas exposed to sunlight, I consider whether it may signal a photosensitivity reaction based on a medication that makes the skin more vulnerable to the sun's rays. If I see atopic dermatitis (also known as atopic eczema, a rash) on an itchy child, I inquire about the rest of the common triad-allergy and asthma. If I notice a ring of warts on the finger of a seven year old, I also look at his or her lips to see if hand-to-mouth behavior has resulted in the spread of the condition via auto-inoculation. If, when I inspect the back of a nervous character, I note that easier to reach areas have multiple scratch marks in different stages of healing amid a forest of erratic depigmentation, but that the harder to reach center of the back is untouched, I shift my diagnostic weight to self-induced neurodermatitis, pathologic skin picking in the absence of any underlying skin disease.
Some of these skin diseases may have protective utility. Psoriasis, for example may have a hidden adaptive function that carries a genetic survival advantage. If the same genes that trigger psoriasis also control the intensity of response to bacterial invasion, then perhaps the combined one-two punch of an enhanced inflammatory response and thickened keratin layer gave those with the psoriasis-predisposing genome a survival advantage. The natural process of desquamation, where the skin rids itself of excess layers of keratin, is heightened in psoriasis and may provide a helpful response to discourage colonization of the skin's surface by undesirable microbes and maintain integrity of the skin by shedding faster than colonization can get traction.
Other protective roles for psoriasis can be seen with cutaneous tuberculosis, a disease that can bring on horrible facial destruction. Psoriasis first came to widespread attention in the medical community in the mid-nineteenth century, coincident with a high prevalence of cutaneous (as well as systemic) tuberculosis. As many researchers have reported, cases of patients with both skin diseases were essentially absent. It may be that psoriatic carriers are protected from tuberculosis, or have a survival edge against the more disfiguring cutaneous tuberculosis. If the psoriasis carrier could be protected against tuberculosis, the predisposing psoriasis genotype could survive. Trials and research with the new biologic drugs for psoriasis have proved that psoriasis patients have highly activated immune systems. And one of the main contraindications for the use of any psoriasis-halting biologic is an active systemic tuberculosis infection-in this case, healing patients' psoriasis might actually worsen their tuberculosis.
Dermatology includes pattern recognition as a primary detection tool, and changes in morphology and distribution of lesions on the body are part of each exam. New growths arise and old ones change form, and infections like candidiasis, from a yeast, erupt in locations where and when they find an opportunity for survival. In a similar fashion, you may notice during a walk in the woods a new flower blooming or pay attention to which species of bird frequents a certain oak or elm. Or you may come across a tree down on the side of the trail and decaying, and note that the way the bracket fungi are oriented on the trunk tells you how long the tree has been non-vertical. As with each observation in nature, ecological abnormalities involving changes on the skin due to invasions, disturbances, and imbalances are an integral part of observing skin diseases and providing treatments.
The skin is an amazing, versatile, organ and science discovers more about its magic every day. As you read on, you will understand more about the wonders of our covering and what can happen when it gets violated or reflects an underlying problem. You may even pick up a clue or two on how to save your skin.