3.5: Epidermal Accessory Structures - Components and Age Changes

There are many places in the skin where groups of epidermal cells have sunk into the underlying dermis so that they may form additional helpful structures. Two of these structures are hair and nails, which will be discussed here because they are visible on the surface of the skin. The others (i.e., sweat glands, sebaceous glands) will be discussed as part of the dermis because they are under the epidermis proper.

Hair

Recall that the upper layer of the epidermis consists of a thin layer of keratin. Hair is also made of keratin. Each hair is formed at the bottom of a deep pit of epidermal cells called a hair follicle, which extends down into the dermis (Figure 3.1). At the base of the follicle, the same processes that produce the stratum corneum occur, leaving the keratin behind as the shaft of the hair.

Each follicle is not always making hair. On a fairly regular basis, the production of cells in a follicle slows and may even stop. When this occurs, the hair falls out and the follicle enters a resting period. After a while the follicle will begin producing cells and melanin again, and a new hair will emerge.

Hair is found on almost all parts of the skin. In areas such as the forehead, it is sparse, thin, and light in color and has only a slight value. However, the scalp, eyebrows, and eyelids have dense, thick, and long hairs that contribute substantially to a person's well‑being in a variety of ways.

Perhaps the most obvious is its cosmetic value. To appreciate this, one need only notice how much time, energy, attention, and money people spend on their hair. A person's appearance has great social, psychological, and economic impact on the quality of his or her life; this holds true for the elderly as well as the young.

Hair also has several biological functions. For example, scalp hair shades the head from sunlight, provides a thermal insulating layer, and cushions the head against bumps. The hair around the eyes shades them and filters out dust and other small particles. The hair in the openings to the nose and ears also serves as a filter.

In addition, hair helps increase the skin's sensitivity to touch. Since hairs jut out from the surface of the body, any object or material that is about to touch the skin or is moving along its surface collides with these hairs. When such collisions move a hair, its motion travels down the shaft to nerve endings around the follicle. These nerve endings detect the motion and send impulses to the brain, informing the person of the presence and motion of the object or material. Recall that such monitoring is the first step in the negative feedback processes necessary for healthy survival. The person can then take the necessary steps to avoid or remove the object or material. Alternatively, if the object or material causing the motion is not harmful, the person may derive pleasure from the sensations, such as those from a caress or a gentle breeze.

Age Changes in Hair

Aging results in four changes that decrease the amount of visible hair. First, there is a decrease in the number of follicles, which decreases the total number of hairs present. Second, increasing proportions of the remaining follicles spend longer periods in the resting stage. This further reduces the amount of hair, since follicles have no hair present during the resting stage. Third, when follicles reenter the active stage, they produce hair more slowly, and so it takes more time for a new hair to emerge. Fourth, almost all the hairs produced are thinner‑related decreases in the levels of sex hormones are the main reason for the decline in armpit and pubic hair. However, relatively high levels of male sex hormones (e.g., testosterone) cause more rapid loss of hair from the scalp. Since aging men retain relatively high levels of sex hormones, they lose much scalp hair. Furthermore, men who have inherited the genes for pattern baldness lose increasing amounts of hair from the crown of the head. Women also have some male sex hormone, which is produced by the adrenal gland. Since the level of male sex hormone in women is low before menopause, loss of scalp hair in women is low at first. After menopause, this loss increases dramatically because menopause is accompanied by a rise in male sex hormone.

While a decrease in both the amount and thickness of hair occurs in most areas of the body, some exceptions occur. In aging women these include an increase in facial hair and thickening and lengthening of some hairs on the chin and upper lip. In aging men, thicker and longer hairs are produced in the eyebrows, on the external ears, and within the ear canals and nostrils. All these alterations can be cosmetically troublesome.

Other cosmetically important changes include the development of air pockets within hairs and decreases in the amount of oil secreted onto the hair, resulting in a loss of softness and luster. In addition, the number of melanocytes in each follicle declines, resulting in a decline in the intensity of hair color. As more follicles lose all their melanocytes, increasing numbers of hairs become white. With declining pigment in each hair and fewer hairs containing any pigment, the hue of a person's hair becomes gray and finally white.

The time and rate of graying are determined mostly by genes. Both the time of onset and the rate of graying of scalp hair are not well correlated with chronological age, and graying of scalp hair shows wide variation among individuals. Therefore, gray hair is a very poor indicator of chronological age. By contrast, the initiation and progress of graying of axillary hair are very good indicators.

The consequences of age changes in hair vary. The amount of cushioning provided for the head remains high. Shading of the eyes, and the filtering action and the contributions to touch sensation hair provides, may improve when hairs thicken and lengthen. By contrast, the decline in the abundance of hair results in decrements in shading and thermal insulation for the scalp. However, wearing a hat can provide the same type of protection for the scalp.

Because of decreases in both the number and length of hairs in most areas, there are widespread reductions in the contributions hair makes to touch perception. This reduction is exacerbated by the decline in both the number and functioning of sensory nerve cells.

All these biological effects may seem slight compared with the variety and degree of social, psychological, and economic effects caused by the appearance of becoming old. While there may be some positive effects from appearing to be older or more mature, most of the effects are negative.

Nails

Like hair, fingernails and toenails are made of keratin produced by essentially the same process as is the keratin in the stratum corneum and in hair. However, no melanin is incorporated into nails.

Nails serve primarily to protect the fingers and toes from traumatic injuries such as crushing, cuts, and scrapes. They can also be used like tools to pick up small objects or scratch irritants off the skin. In addition, though the toenails are usually hidden from view, the fingernails are usually very visible and therefore can have a significant impact on a person's appearance.

Age Changes in Nails

As a person ages, the rate of growth of nails decreases by as much as 50 percent and the thickness and strength of the nails also decrease. The keratin plate becomes less clear, longitudinal grooves develop, and the growth zone at the base of the nail decreases in size. Though these changes are primarily due to aging, they can also be caused by trauma and reduced blood flow to the extremities. Changes are greater in the toenails than in the fingernails because there is a greater age‑related decline in the blood supply to the feet compared with the hands.

Age changes in nails have several undesirable consequences. The structural weakening of the nails makes them susceptible to injury and disfigurement. The declining growth rate means that the damage is present for a longer period before the injured part grows out and is worn off or cut away. Therefore, nails are less able to perform their functions. Furthermore, because of the higher incidence, severity, and duration of nail injuries, fungal infections of the nails become more common.

Such infections cause the nails to thicken, become opaque, and become misshapen. Infected nails may become unsightly, causing cosmetic problems. In addition, curing fungal nail infections takes longer because a declining blood supply to the nails causes medications to be delivered more slowly. Eventually fungal infections of the toenails may become impossible to cure.

Aging and disease changes in the toenails can have a more serious biological impact than can those in the fingernails because the toenails are out of sight most of the time and therefore often do not get proper care. Furthermore, because of age changes in the eyes and the skeletal system, it is increasingly difficult for aging individuals to see and reach their toenails, resulting in further decrements in toenail care. For example, toenails may become so large that they interfere with the proper fit of shoes, making walking difficult or painful. Toenails are also common sites of infection in diabetics.