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This child has xeroderma pigmentosum. Sun exposure causes extreme freckling, and skin cancer is likely to develop because he lacks DNA repair enzymes. The large lesion on his chin is a skin cancer.

Fingerprints form from these undulations of the skin at the distal end of the palmar surface of a finger. Fingerprints are used for purposes of identification because they are individually unique. The pattern of a fingerprint is genetically determined, and the prints form during fetal existence. However, during a certain time early in development, fetal movements can change the print pattern. Because no two fetuses move exactly alike, even the fingerprints of identical twins are slightly different.

The dermis binds the epidermis to the underlying tissues. It is largely composed of irregular dense connective tissue that includes tough collagenous fibers and elastic fibers in a gel-like ground substance. Networks of these fibers give the skin toughness and elasticity. On the average, the dermis is 1.0-2.0 mm thick; however, it may be as thin as 0.5 mm or less on the eyelids or as thick as 3.0 mm on the soles of the feet.

The dermis also contains muscle fibers. Some regions, such as the skin that encloses the testes (scrotum), contain many smooth muscle cells that can wrinkle the skin when they contract. Other smooth muscles in the dermis are associated with accessory organs such as hair follicles and glands. Many striated muscle fibers are anchored to the dermis in the skin of the face. They help

Shier-Butler-Lewis: II. Support and Movement 6. Skin and the © The McGraw-Hill

Human Anatomy and Integumentary System Companies, 2001

Physiology, Ninth Edition variety of colored areas — variegated brown, black, gray, or blue. A melanoma usually has irregular rather than smooth outlines (fig. 6B).

People of any age may develop a cutaneous melanoma. These cancers seem to be caused by short, intermittent exposure to high-intensity sunlight. Thus, risk of melanoma increases in persons who stay indoors but occasionally sustain blistering sunburns.

Light-skinned people who burn rather than tan are at higher risk of developing a cutaneous melanoma. The cancer usually appears in the skin of the trunk, especially the back, or the limbs, arising from normal-appearing skin or from a mole

(nevus). The lesion spreads horizontally through the skin, but eventually may thicken and grow downward into the skin, invading deeper tissues. Surgical removal during the horizontal growth phase can arrest the cancer. But once the lesion thickens and spreads into deeper tissues, it becomes more difficult to treat. An experimental gene therapy injects genes directly into melanoma cells. These genes direct the cells to produce surface proteins that attract the immune system to attack the cancer.

The incidence of melanoma has been increasing rapidly for the past twenty years. To reduce the chances of occurrence, avoid exposure to high-intensity sunlight, use sunscreens and sunblocks, and examine the skin reg ularly. Report any unusual lesions — particularly those that change in color, shape, or surface texture—to a physician.

Replacements for natural sun-tanning may be ineffective or dangerous. "Sunless tanning agents" do not tan the skin at all, but merely dye it temporarily. "Tan accelerators," according to manufacturers, are nutrients that supposedly increase melanin synthesis in the sunlight, but there is no scientific evidence that they work. Tanning booths may be dangerous. Even those claiming to be safe because they emit only partial ultraviolet radiation may cause skin cancer.

Enjoy the sun—but protect yourself! ■

Mole Skintag

produce the voluntary movements associated with facial expressions.

Nerve fibers are scattered throughout the dermis. Motor fibers carry impulses to dermal muscles and glands, and sensory fibers carry impulses away from specialized sensory receptors, such as touch receptors (see fig. 6.2).

One type of dermal sensory receptor, Pacinian corpuscles, is stimulated by heavy pressure; whereas another type, Meissner's corpuscles, senses light touch. Still other receptors respond to temperature changes or to factors that can damage tissues. Sensory receptors are discussed in chapter 12. The dermis also contains blood vessels, hair follicles, sebaceous glands, and sweat glands, which are discussed later in the chapter.

Subcutaneous Layer

The subcutaneous layer (hypodermis) beneath the dermis consists of loose connective and adipose tissues (see fig. 6.2). The collagenous and elastic fibers of this layer are continuous with those of the dermis. Most of these fibers parallel the surface of the skin, extending in all directions. As a result, no sharp boundary separates the dermis and the subcutaneous layer.


Figure 6.5

(a) Transmission electron micrograph of a melanocyte with pigment-containing granules (10,000x). (b) A melanocyte may have pigment-containing extensions that pass between epidermal cells.

The adipose tissue of the subcutaneous layer insulates, helping to conserve body heat and impeding the entrance of heat from the outside. The amount of adipose tissue varies greatly with each individual's nutritional condition. It also varies in thickness from one region to another. For example, adipose tissue is usually thick over the abdomen, but absent in the eyelids.

The subcutaneous layer contains the major blood vessels that supply the skin. Branches of these vessels form a network (rete cutaneum) between the dermis and the subcutaneous layer. They, in turn, give off smaller vessels that supply the dermis above and the underlying adipose tissue.

Subcutaneous injections are administered through a hollow needle into the subcutaneous layer beneath the skin. Intradermal injections are injected within the skin. Subcutaneous injections and intramuscular injections, administered into muscles, are sometimes called hypodermic injections.

Some substances are administered through the skin by means of an adhesive transdermal patch that includes a small reservoir containing a drug. The drug passes from the reservoir through a permeable membrane at a known rate. It then diffuses into the epidermis and enters the blood vessels of the dermis. Transdermal patches are used to protect against motion sickness, chest pain associated with heart disease, and elevated blood pressure. A transdermal patch that delivers nicotine is used to help people stop smoking.

What kinds of tissues make up the dermis?

What are the functions of these tissues?

What are the functions of the subcutaneous layer?

Accessory Organs of the Skin

Accessory organs of the skin extend downward from the epidermis and include hair follicles, nails, and skin glands. As long as accessory organs remain intact, severely burned or injured dermis can regenerate.

Hair Follicles

Hair is present on all skin surfaces except the palms, soles, lips, nipples, and parts of the external reproductive organs; however, it is not always well developed. For example, hair on the forehead is usually very fine.

Each hair develops from a group of epidermal cells at the base of a tubelike depression called a hair follicle (har fol i-kl). This follicle extends from the surface into the dermis and contains the hair root, the portion of hair embedded in the skin. The epidermal cells at its base are nourished from dermal blood vessels in a projection of connective tissue (hair papilla) at the deep end of the follicle. As these epidermal cells divide and grow, older cells are pushed toward the surface. The cells that move upward and away from the nutrient supply become keratinized and die. Their remains constitute the structure of a developing hair shaft that extends away from the skin surface. In other words, a hair is composed of dead epidermal cells (figs. 6.6 and 6.7). Both hair and epidermal cells develop from the same types of stem cells.

Hair shaft

Arrector pili muscle


Hair follicle

Region of — cell division

Basement membrane

Hair shaft

Region of — cell division

Basement Membrane Stem Cells

Basement membrane

Hair papilla

Hair papilla

Follicle Regions

Dermal tissue Hair follicle

Hair root

Region of cell division

Hair papilla

Adipose tissue

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