What are nails?

Structure and Functions

Nails function to protect fingers and toes against bumps and trauma. Fine touch is amplified and skillful manipulation of small objects with the fingers is enabled by the presence of nails. Nails also provide the ability to scratch, both as a temporary relief of an itch and in personal defense. Nails are important social communicators of beauty and sexuality and hence are the focus of a major cosmetic industry.

Biologically, nails are characterized as plates of tightly packed, hard epidermis cells filled with a protein called keratin. Nails are normally seen on the dorsal side (the side opposite the palm or sole) of all fingers and toes. The anatomy of the normal nail consists of a nail plate, proximal nail fold, nail bed, matrix, and hyponychium. These components are epithelial-derived structures, like skin and hair, which emerge from the live germinative zone of the epidermis. In nails, these cells differentiate and form the horny layer, which is considered to be dead.

The nail plate is a relatively hard and flat, transparent, horny structure that is rectangular in shape. It rests on the underlying nail bed but typically extends beyond the bed as an unattached, free-growing edge reaching toward or beyond the tip of the finger or toe. On the fingers, the thickness of the plate in adults increases from about 0.7 millimeters at the proximal edge to about 1.6 millimeters at the distal edge. The terminal tip thickness varies considerably between persons.

Normally, a pinkish nail bed is seen through the transparent nail plate. Frequently in the thumbnail and sometimes in the other fingernails, a whitish, semi-moon-shaped structure called the lunula is seen that extends under the proximal nail fold. The borders of the nail plate are covered by skin structures: two lateral folds and a single proximal fold.

The proximal nail fold is the cutaneous or skin structure that is in continuity with the visible proximal border of the nail but overlies part of the nail root. The ventral side (underside) of the proximal nail fold provides physical protection to the germinative zone of the nail and aids in the physical attachment of the nail plate. About a fourth of the total surface area of the nail plate is located under the proximal fold. The cuticle is a layer of epidermis extending from the proximal nail fold and attached to the dorsal side of the nail. The cuticle functions to provide a physical seal against microbes and chemical irritants, which may otherwise enter the matrix and affect nail production.

The nail bed is the portion of the digit upon which the nail rests. The nail bed is highly vascular, with numerous capillaries. It consists of epithelial tissue and extends from the lunula to the point where the bed separates from the nail. A series of fine longitudinal folds in the nail bed corresponds to the undersurface structure of the nail. This arrangement enhances the adherence of the nail plate to the nail bed.

The nail matrix is generally considered the most proximal part of the nail bed and is bordered by the proximal nail fold. On the distal end it is bordered by the distal margin of the lunula. The nail matrix epithelium cells consist predominantly of keratinocytes in both a basal and a spinous layer. Melanocytes and Langerhans cells are intermingled with keratinocytes. It is within the matrix that the germinating center of nail growth is found. Basal epithelial cells increase in number through mitosis, or division, and then differentiate into keratinocytes, which are epithelial cells filled with keratin protein. These keratinocytes condense their cytoplasm, lose their nucleus, and form flat, horny-looking dead cells. As further cell division occurs in the nail matrix, more distal keratinocytes are pushed out to form the nail plate.

The hyponychium consists of epidermis tissue that underlies the edge of the nail plate and extends from the nail bed to the distal groove. It functions to provide a defense against entry of bacteria under the free edge of the nail plate. Excessively vigorous cleaning may damage the hyponychium and allow for bacteria to enter more readily under the nail plate.

The turnover rate of matrix cells determines the growth rate of the nail. This rate varies with age, environmental conditions, nutritional status, and the specific digit. The growth rate proportionately increases with the length of the digit; thus the middle finger nail grows the fastest while the growth rate in the little finger is the slowest. Fingernails grow three times as rapidly as toenails. The growth rate is more rapid in the winter than in the summer. Furthermore, nails grow faster in young children than in older adults. It takes about six months for a fingernail to completely grow in. Male nails grow faster than female nails. Nails on the dominant hand grow faster than those on the other hand.

If nails are protected and untrimmed, they can grow to considerable lengths. Such long nails were prized by the wealthy classes in imperial Chinese culture as an indication of status. The practice of painting toenails red may have originated in the Ottoman seraglio, where it was a signal of menstrual status.

Nails continue to grow throughout life without a resting phase. Contrary to folk belief, nails cease growing when an individual dies. The matrix cells stop producing deoxyribonucleic acid (DNA) and dividing soon after death, and thus the nail bed cannot grow longer. The appearance of nail growth after death is due to a retraction or shrinkage of nail matrix tissue, resulting in the apparent lengthening of the nail plate.

Professional grooming of the nails for both men and women is termed a manicure. Manicure procedures include cutting the nails according to fashion standards to improve their cosmetic appearance. A pedicure is the term applied to grooming the toenails. Typically, the nails are first soaked in a soapy solution to soften the nail plate and to remove dirt and debris. It is often fashionable to trim the nails to a delicate arc at the middle of the fingertip. The corners of the nail are typically filed. While this shape is attractive in creating the illusion of longer, slender fingers, it heightens the probability of nail plate fractures, hangnails, or ingrown nails. The cuticle, considered to be unattractive by manicurists, is typically minimized, partially removed, or traumatized. This may increase the incidence of fungal invasion and disease. Most of the problems associated with a manicure arise from excessive manipulation of the cuticle.

Nail polish typically consists of pigments suspended in a volatile solvent that also contains a film-forming agent. When the polish is applied to the finger, a covering film develops over the nail. The film is permeable to oxygen, which allows gas exchange to occur between the atmosphere and the nail plate. Resins and plasticizers are added to the polish to increase the flexibility of the film and to minimize chipping. The variety in nail polish color is due to the addition of coloring agents. Deep red nail polishes can cause a temporary yellowish staining of the nail plate.

Nail adornments are sometimes used. Frequently, small nail jewels or ribbons are applied to the fingernails immediately before the nail polish dries, allowing the decoration to adhere to the nail plate. Since some people frequently develop contact dermatitis in reaction to nickel, gold or nickel-free jewels should be selected. Artificial nail tips made of plastic also are glued to the nail tip to create the illusion of an elongated natural nail tip. The gluing may cause nail problems, since a portion of the natural nail is occluded by the glue. This occlusion inhibits oxygen transfer and stresses the nail plate. Frequently, the nail may thin and be unable to support its own weight after the plastic tip is removed. Removal of the plastic tip may result in nail pitting.

Disorders and Diseases

Nails are useful indicators of skin disease and internal disorders. An abnormally pigmented band in the nail may indicate a malignant melanoma. A yellow nail may indicate psoriasis or a fungal infection. Pulmonary disease or smoking may also cause yellow or brownish nails. Antimalarial drugs may cause the nail to darken in appearance. Frequently, psoriasis causes pitting of nails and an acceleration in their growth rate. Chronic chest disease or a cyanotic congenital heart disorder is frequently associated with club-shaped deformity of the nail plate. Beau’s lines, which are transverse depressions in the nail plates, are associated with illnesses such as coronary thrombosis, pneumonia, and severe injuries. Drug treatment may cause nail breakdown or destruction or complete shedding of the nail plate.

An unexplained aspect of nail physiology is its relationship to lung physiology. Hippocrates first described a connection between lung parenchymal disorders and an edema in the connective tissue beneath the lunula that results in clubbing. The relationship has long been recognized, but the causal link is still unknown.

An ingrown nail
results when a deformed nail grows improperly into the skin or when the skin around the nails engulfs part of the nail. Wearing narrow, tight shoes can cause or worsen this pathology. Initially, symptoms may be slight or mild, but with time may come increasing pain. The affected area becomes reddish and, if not treated, may become infected. If infected, the area becomes swollen, inflamed, and painful. Blisters may develop. Treatment involves trimming away the nail from the infected area, allowing the inflammation to decline and the area to heal.

Clubbing is a disorder characterized by a bulblike enlargement of the nail with increased horizontal and longitudinal curvatures. Clubbing involves both fingers and toes and commonly begins at puberty. The disorder may be genetically inherited or acquired. Clubbed nails often have a spongy feel when pressure is placed on the proximal nail fold. This is due to the expanded soft tissue that underlies the nail. Acquired clubbing is often associated with another clinical pathology, most commonly pulmonary or cardiovascular disease. However, at times it is also associated with gastrointestinal inflammatory disease or cystic fibrosis.

A fungal infection is the cause of onychomycosis, which is the most common nail disorder. About half of all nail problems can be linked to this disease, which affects as many as 15 to 20 percent of people in North America. It is uncommon in children and more frequently seen in aged adults.

Several fungi may cause onychomycosis, although most belong to the group called dermatophytes. The fungi are present in soil, and indirect transmission to humans frequently occurs through public swimming pools and shower floors. Some yeasts and molds are also thought to cause this clinical condition. Alternatively, this infection is associated with athlete’s foot infection. Typically this condition is found in toenails more frequently than fingernails. The presence of the fungus is further evidenced by scaling on the plantar surface of the foot, where it is often harbored.

In onychomycosis, the fungal organism invades the nail bed, causing a progressive change in the color, texture, and structure of the nail. The nail may turn white, thicken, and even detach from the nail bed. Debris from the infected nail often collects under its free edge. If untreated, the pathology may involve the entire nail plate, and rarely will the nail unit spontaneously heal itself.

Since several organisms may induce the pathology, effective treatment depends on matching the curative with the causal agent. Topically applied antifungals are seldom effective because most cannot penetrate the nail-plate barrier to reach the causal organism. Systemically (orally) administered antifungal therapy frequently uses one of several drugs such as griseofulvin, thiabendazole, or ketoconazole. These drugs are generally effective in halting further invasion of the fungus. While treatment continues, the new nail growth is usually normal, so treatment typically continues until the old nail is replaced by new nail and is then discontinued.

More than seventy tumors have been associated with parts of the nail. They may originate from the epidermis, dermis, subcutaneous tissues, or bone and may be found in the nail bed, nail matrix, hyponychium, or nail fold. The tumors may take various forms, including warts, erosion or ulceration of the nail bed, malignant neoplasms from underlying melanocytes, benign fibromas of the connective tissue, or squamous cell carcinomas. Diagnosis usually is made by taking a biopsy of the affected area. Treatment typically involves surgical removal of the tumor.

Perspective and Prospects

The earliest cellular growth leading to nail formation can be seen histologically at eight weeks of human development, the end of the embryonic period. Microscopically, the cells forming the proximal edge of the nail field and the future matrix can be distinguished at this time. The earliest gross anatomical appearance of nails is seen on the finger digit surface at about nine weeks of development, the very early fetal stage. By eleven weeks of age, the nail field is seen clearly on the hand digits of the fetus. By twenty weeks of age, the fetus shows a nail plate and bed and a proximal cuticle. By thirty-two weeks, the third trimester of pregnancy, adult-type nail structures are visible in the fetus, including a nail plate, matrix, and bed and a forming hyponychium.

Aging results in changes and disorders in nails. When people get older, the color, contour, growth rate, surface texture, and thickness of the nail plate change. Some disorders are more prevalent with aging, such as brittle nails, splitting or fissuring of the nail plate, and increased infections. Aged nails appear dull and opaque, with their color varying from yellow to gray. Frequently in older persons, the lunula is decreased in size or is absent. The growth rate of nails decreases with aging. The most rapid period of growth is during the first thirty years, after which the rate steadily declines. Nail plate thickness frequently increases with advanced age, in combination with discoloration and loss of translucency of the nail plate.

For many years, blood and urine specimens have been used to detect and measure body concentrations of therapeutic drugs or drugs of abuse. During the past decade, alternative biological specimens such as nails and hair have been frequently used as the basis for drug detection. The basis for drug detection in nail clippings is that the dividing epidermal cells that form the nail plate also incorporate drugs from the systemic circulation. The subsequent cornification of these cells traps the drug within the forming nail plate. Drug-detection methods involve taking a sample of nail clippings and extracting and identifying drug molecules via immunochemical or chromatographic techniques. These techniques are extremely sensitive and capable of detecting minute quantities of drug in the samples; as little as ten milligrams of nail clippings is required to detect the presence of drugs. Twenty-first-century drug-screening methods have detected amphetamines and cocaine in nail clippings. Using nail clippings for drug analysis and screening provides a long-term measure of drug exposure that may potentially represent months of drug use. Furthermore, nail clippings are relatively easy to collect and involve a noninvasive procedure; samples are easily stored, and once incorporated in the nail tissue, most of the drugs are presumably stable.

Modern medicine in the early twenty-first century still lacks adequate descriptive science as well as understanding of the molecular mechanisms that control nail development and growth. To date, the specific genes or gene products that initiate nail growth have not been identified. The molecular basis for brittle nails, clubbing, and other nail pathologies is not known. Is onychomycosis affected by a systemic immune deficit? What causes yellow nail syndrome? Answers to these questions as well as additional information about the molecular control of nail physiology will greatly increase understanding and lead to better treatments for nail disorders.

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