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 fingernails.
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, may contribute to 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, premalignant, 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 precancers—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.
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