What are skin disorders?

Anatomy of the Skin

The skin is the largest organ of the body. It provides a barrier between the external world and the internal world: It protects against external contamination and helps to maintain the sterility of the internal body. The skin also assists in temperature regulation; humans can survive only within a narrow temperature range. The skin has nerve receptors that supply the brain with information, providing an interface with the world. There are specialized receptors for touch, temperature, vibration, and position in space (proprioception).

Appendages to the skin are fingernails, toenails, and hair. They are mainly of psychological importance. Nails protect the tips of fingers and toes in humans but are not needed for protection as claws are in lower animals. Hair is analogous to feathers. In birds, tiny muscles attached to the base of each feather cause them to be ruffled; this creates air pockets and allows birds to conserve heat and keep warm. The same muscles persist in humans, causing “goose flesh,” but they do not serve any other function. The main importance of these appendages is cosmetic. For example, people spend billions of dollars on hair care products each year. The motivation for this activity is psychological.

The two main layers in skin are the epidermis and the dermis. The epidermis is the upper or outermost layer, and cells are continually formed at its base. As new cells are formed, existing cells are pushed toward the surface of the skin. These cells gradually lose their watery central contents, causing them to dry out (desiccate) and become flattened. This process normally spans approximately a month. Thus, the surface of the body is largely composed of dead cells that have become flattened. These cells are normally lost on a continual basis and create dandruff when shed from the scalp. On other parts of the body, sloughed cells provide excellent conditions for bacterial growth, accounting for the unpleasant odors that accompany poor hygiene habits.

Two other important types of cells are found in the epidermis: melanocytes and Langerhans cells. Melanocytes contain melanin and provide all the variations of pigmentation found in the human species. They multiply when stimulated by the ultraviolet radiation in sunlight. This causes the skin to become darker, a protective mechanism against damage from ultraviolet radiation. Langerhans cells contain surface receptors for immunoglobulins. They play a central role in allergic reactions of the skin, such as contact dermatitis or delayed hypersensitivity reaction.

The
dermis is an inner layer of skin located beneath the epidermis. Its main function is protection. Within the dermis are highly specialized cells containing microscopic filaments. These cells impart tensile strength to the skin in much the same way that fibers strengthen fiberglass or reinforcing steel mesh strengthens concrete. Because they are so dense, they also serve as a barrier to the entry of most pathogens and many chemicals. Eccrine sweat glands are found in the dermis throughout the entire body. These produce a salty secretion (essentially salt water) that assists in
thermoregulation through evaporative cooling. They are also sensitive to emotional stress. Apocrine sweat glands are primarily in the armpits (axilla) and the groin, and they produce a milky secretion. When these secretions are broken down by bacteria on the surface of the skin, a characteristic odor is produced. The bases of hair follicles are also found in the dermis. The small sebaceous, or oil-secreting, gland associated with most hair follicles has the function of softening and moisturizing the hair.

Hair is found on most surfaces of the body; exceptions are the palms of the hands, the soles of the feet, and the glans penis in men. The texture and length of the hair vary with location on the body, gender, genetic heritage, and age. Dramatic increases in the growth and distribution of hair occur at puberty. With increasing age, hair is typically lost from the scalp and other body parts. It also changes color, assuming a gray or white color because of the loss of melanin at the base of the hair follicle.

Complications and Disorders

When normal skin anatomy and physiology are upset, several common diseases or disorders result. When the barrier provided by the skin is broken, bacteria, viruses, fungi, and other pathogens can invade the body, leading to infections. Locally, these infections can cause inflammation (redness and pain) of the skin; if widespread, they can lead to systemic infections. When the cells and other substances found in the skin become irregular or are abnormal, skin disorders or conditions result.

Skin disorders and conditions.

Pigmentation of the skin results from the presence of melanocytes, cells that manufacture and contain melanin. Most humans have pigmentation over their entire bodies; the degree of pigmentation varies with different racial and ethnic groups. Local areas of increased color have a range of names depending on the size of the pigmented area. A freckle is small and discrete. A nevus (mole) is a larger area of hyperpigmentation. These conditions are attributable to underlying variations in the distribution of melanocytes. They are genetic in origin and permanent; they are also accentuated by exposure to sunlight. Melasmas are irregular, flat, light brown areas on the neck, cheeks, or forehead. They are caused by hormonal changes associated with pregnancy or contraceptive pills and by exposure to sunlight. Melasmas fade with the reduction of excess hormones. There are also color changes in the labia of females during pregnancy; these changes are both harmless and permanent.

Generalized increases in skin coloration can occur with some metabolic diseases. Addison disease
involves an increase in melanocyte-stimulating hormone. This leads to an overall bronzing of the body, with accentuation in creases of the palms and soles. The condition subsides with treatment of the underlying cause of the disease. Similar pigment increases are associated with some forms of lung cancer, hemochromatosis, and chronic arsenic exposure. The latter two conditions are caused by the deposition of iron (hemochromatosis) and arsenic in the skin.

Generalized decreases in skin coloration can also occur. If melanocytes fail to migrate to the skin during embryologic development, hair follicles will lack color, resulting in a condition called piebaldism. Characteristically, this is a white patch in the hair of the forehead.
Vitiligo is caused by an immunologically mediated loss of melanocytes. Individuals with Phenylketonuria (PKU)
experience a generalized depigmentation of hair and eye color, in addition to intellectual disabilities, if the condition is not adequately and promptly treated. An individual totally lacking melanocytes is called an albino. The loss of hair is called alopecia. It can occur because of aging, sustained pulling on the hair with some hairstyles, and genetics. Women do not usually experience much
alopecia until after the menopause. Conversely, some men start to lose their hair during their twenties.

Skin diseases.
Eczema
or
dermatitis is a general term that describes a skin disease involving vesicles that ooze fluid. These conditions are usually characterized by a rash; they are inflammatory reactions, commonly caused by contact with a chemical or plant material. They can be caused by an adverse reaction to a drug or by sunlight. Bacteria, yeasts, or other fungi on the skin can cause infections; one common example, often mistaken for eczema, is althete's foot. Most rashes itch or burn; they can be spread by scratching.

Maculopapular diseases encompass several common skin conditions, such as red measles (rubeola), German measles (rubella), and scarlet fever. Viruses that land on the skin cause these diseases. They are characterized by relatively large, localized areas of changed skin color (macules) that are also raised (papules) but not fluid-filled. After their clinical course is run, they disappear without leaving a scar. The more dangerous toxic shock syndrome also belongs to this group of diseases; it is caused by toxin from the bacteria Staphylococcus aureus.

Thickening of the skin and the formation of red to purple areas having sharply defined borders characterize papulosquamous skin diseases. The most common example is
psoriasis. Other examples are pityriasis and ichthyosis. The pathology responsible for psoriasis is an alteration in the normal development of skin cells. In individuals with psoriasis, new skin cells develop and migrate to the surface in only five days instead of the usual thirty. This fact alone explains the flaking (rapid cell turnover), redness (thinner skin and a rich blood supply for new skin), and pain and itching (less protection for sensory nerve endings) experienced. Pityriasis includes a group of different conditions caused by different viruses. Patches or large spots develop on the skin. They usually resolve within a few weeks. Aside from being locally photosensitive, they usually are not serious. Ichthyosis describes a group of genetic
conditions characterized by extreme scaling of the skin.

Vesiculobullous diseases have fluid-filled blisters that can vary in size from relatively small (vesicles) to relatively large (bullae). Insect bites, herpes, and some bacterial infections lead to the formation of vesicles or bullae. Such conditions are attributable to an immune reaction that leads to the formation of blisters at the junction between epidermis and dermis. They can be accompanied by intense pruritus (itching); scratching often leads to scarring.

Pustular diseases of the skin include
acne, folliculitis, and candidiasis. They are characterized by the inflammation of hair follicles caused by surface bacteria or yeasts. Adequate personal hygiene is the most effective method of prevention. These diseases are usually not serious, but prolonged or repeated attacks can result in scarring and disfigurement. The sebaceous glands, which secrete oil at the base of hair follicles, can increase in size. The subsequent increase in oil output worsens the condition.

Clogged sweat glands can lead to acne. While this is primarily a problem for teenagers, it can affect individuals of any age. Exposure to cutting oils and other hydrocarbons such as gasoline and paint thinners can cause a similar condition called chloracne, which is inflammation in the base of hair follicles found on exposed skin in areas such as the nape of the neck, forearms, and face. The inability to sense temperature and regulate body heat through sweating is called anhidrosis, a condition that can cause shock and potentially death.

Other diseases that can affect the skin. Five such diseases are worthy of mention: leprosy, scleroderma, lupus, atherosclerosis, and diabetes mellitus. Leprosy, or Hansen’s disease, is caused by infection by Mycobacterium leprae, a relative of the bacteria that cause tuberculosis. In leprosy, the causative organism accumulates in the skin and peripheral nerves. This causes disfigurement and loss of sensation, the latter being similar to that experienced by an uncontrolled diabetic. Disfigurement is responsible for the stigma associated with leprosy since ancient times: loss of fingers and toes, as well as mutilation of the nose and ears. Leprosy is caused by long-term association with the organism and can be adequately treated with appropriate antibiotics.

Scleroderma (literally, “hard skin”) is an uncommon disease characterized by fibrosis of the skin and involvement of visceral organs. The skin involvement can range from an isolated, hardened patch to a life-threatening, generalized condition described as an ever-tightening case of steel. The skin becomes stretched tightly over the underlying skeleton. Skin tone is lost with restriction of movement.

Systemic lupus erythematosus is a disease of unknown etiology that is characterized by inflammation in many different organ systems. The skin is usually involved, as nearly all individuals with lupus develop a characteristic butterfly-shaped rash on their faces. This red coloration covers the cheeks and nose. Persons with lupus are also sensitive to sunlight, and many develop alopecia. Most of those affected are female. The disease waxes and wanes; treatment depends on the particular organs involved.

Atherosclerosis and diabetes can block the arteries supplying the nerves of the skin, leading to a loss of sensory input. When the patient is unable to experience pain, cuts and other abrasions on the skin are not noticed. Untreated, these lesions can lead to
gangrene, sometimes requiring
amputation of a body part.

Skin cancer. The most commonly diagnosed form of cancer is that involving the skin. It is not the most fatal form, but millions of cases are discovered annually. The origin of most skin cancers can be traced to excessive exposure to radiation from the sun. They can occur on any surface of the body, although they are more common on areas that are usually exposed to the sun, such as the face, the backs of the hands, and the neck. Skin cancers can arise in the epidermis or dermis. The majority are noncancerous, or benign. Epidermal nodules are characterized by local thickening of the epidermis, often accompanied by scaling of the skin in the affected area. Nodules in the dermis may appear as lumps with no alteration of the epidermis above them.

There are three malignant forms of skin cancer. Basal cell carcinoma
arises from cells deep in the epidermis. This form of tumor rarely spreads (metastasizes), but it can be extensive and destructive locally. Squamous cell carcinoma is less common but can be invasive (involving adjacent tissues) and can metastasize. Melanoma
is relatively uncommon but can grow extremely rapidly; it has the potential to be fatal in a matter of months. It involves the uncontrolled growth of melanocytes. Melanomas have irregular borders and color or pigmentation. Any pigmented lesion or suspicious change in the skin should be evaluated by a medical professional in a timely manner.

Prevention is the preferred method of dealing with skin cancer. When outside, loose-fitting clothing can provide protection from the sun, and a hat can protect the head. When exposure is unavoidable, a product with a sun-blocking agent will reduce exposure. Limiting the time of exposure to the sun until the body has reacted by producing additional melanocytes (tanned) is recommended.

Prolonged exposure to the sun also accelerates changes in the skin associated with aging. Collagen fibers provide the characteristic firm feel to the skin of a young person. With aging the skin becomes less firm, losing some of its tone, and begins to sag. Inadequate moisture also contributes to the loss of skin tone. Excessive exposure to the sun hastens both of these processes.

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