Skin cells come in several types. The epidermis is formed by multiple layers of keratinocytes. These cells make keratin, which is a type of protein that provides the skin with its structural integrity. Keratinocytes make up 90 to 95 percent of the cells in the epidermis.21, 28 The keratinocytes in the outer layer are dead, whereas the keratinocytes in the bottom layer are alive and produce the keratinocytes, which eventually make their way up to the surface of the skin. Keratinocytes also produce hair and nails.
The epidermis also contains melanocytes. There is about one melanocyte for every keratinocyte in the skin.28 Melanocytes are cells that produce a pigment called melanin. Melanin is transferred to the cells of the epidermis and hair, giving skin and hair their color. The number of melanocytes in the epidermis is the same for all races, but the amount of melanin produced varies. Melanin absorbs ultraviolet light and protects us from sun damage.21 The melanin shields the DNA of the nucleus in keratinocytes from the mutating effects of the sun.11, 28
Tanning is the result of increased production of melanin in response to exposure to ultraviolet light and provides greater sun protection.
Other skin cells include Merkel’s cells, Langerhans cells, and fibroblast cells. Merkel’s cells are sensory receptors that respond to sensations of pressure and are more numerous in the palms and soles of the feet. Langerhans cells are found in the epidermis and dermis as well as other parts of the body. They monitor immune reactions in the skin and play an important role in reactions to poison ivy and other skin irritants.28 Fibroblast cells, which produce collagen, are the primary cell type in the dermis.
Skin can reflect an individual’s general state of health. Skin can suggest that a person is tired or ill. A skin problem also can reflect the onset of another disease. For example, skin itches may be harbingers of diabetes and kidney disease. Clear skin is an important aspect of sexual attraction in virtually all cultures. From an evolutionary point of view, the association between good health (and fertility) and unblemished skin may be responsible for our attraction to those with clear skin.
Since skin is important to sexual attraction, it is not surprising that some people modify the appearance of their skin to enhance their attractiveness. The most common methods include tanning, piercing, and tattoos. Unfortunately, all of these practices can have negative and potentially serious health consequences. A recent survey of 454 university students revealed that more than half had a body piercing and about one-quarter had a tattoo.35 Nearly one-fifth reported that they had a medical complication due to the procedure itself or how they cared for the piercing or tattoo afterward.
Research into the science of skin is leading to new understandings about how the skin performs its vital functions. For example, studies are shedding light on how skin senses heat,50 interacts with the immune system in wound repair,25 and elicits responses to antigens presented at the skin surface.22 Other studies are concerned with developing new ways for treating people whose skin has been damaged by accident or disease. Each year, there are about 13,000 hospitalizations in the United States that require extensive skin grafting.58 Unfortunately, the existing skin graft technology has limitations. One company called Stratatech has discovered a rare mutation in a culture of skin cells that allows the cells to grow indefinitely. Tests using animals have shown that skin from this culture can be used successfully to treat wounds. It is hoped that this culture will develop into an off-the-shelf product used by doctors performing skin grafts. Researchers looking at skin stem cells taken from mice have found that they can develop into other types of cells such as nerve, muscle, and fat cells.55 Their intention is to coax human skin cells to form other cell types that could be used to treat patients with a variety of disorders.
The skin is also being exploited in drug-delivery techniques. Such techniques involve widening the skin’s pores using ultrasound waves or an electric shock, or even using a grid of microscopic needles.12 These approaches offer a number of advantages over traditional means of drug delivery. They can ensure the steady release of a drug over long periods of time and bypass the rapid breakdown that occurs in the digestive system. They are also painless and convenient.
Similar to other organs in the body, bone, muscle, and skin rely on interactions with other body parts to function normally.
As discussed in previous sections, bone, muscle, and skin are living systems and are active metabolically. They are connected, as are all other organs, by the body’s cardiovascular system. This allows bone, muscle, and skin to respond to hormones and growth factors produced by other tissues. As a result, growth and other metabolic activities in bone, muscle, and skin occur in a coordinated manner. The nervous system also allows interactions between bone, muscle, and skin. Consider what occurs when nerves in the skin of fingers contact a hot object—the muscles of the arm quickly contract, and the arm is moved away from the heat.
In this section, we consider two examples of how bone, muscle, and skin interact. First, we consider joints, which involve interactions between bones and muscles, and then we look at interactions among all three systems related to vitamin D.
A joint is the place where two bones meet. Because bones are hard, tough structures that resist movement individually, joints form new structures that can move. By joining, or articulating, all the bones of the body, a skeleton of defined shape that is capable of movement is formed.
Joints can be classified in several ways, but for our purposes, joints will be classified by the type of movement they allow. Accordingly, joints can be separated into three main groups: fixed or immovable, slightly movable, and freely movable. Fixed or immovable joints are found between the bones of the skull. The individual bones are joined by dense, fibrous connective tissue, which is why these joints are also called fibrous joints. Fixed joints serve a protective function, although they do allow growth to occur. Teeth are attached to the jaws by fixed joints.
Slightly movable joints are found, for example, between individual vertebrae of the vertebral, or spinal, column and where ribs join to the breastbone. In slightly movable joints, the bones are attached to one another by pads or discs of the connective tissue, or cartilage.
Most joints in the human body are freely movable and are characterized by a cavity that contains a fluid, called synovial fluid, that provides lubrication. The ends of adjacent bones have complementary shapes, which further reduces friction, and are covered with a layer of smooth, hard cartilage. These joints are completely enclosed by a baglike ligament that holds the joint together and prevents the synovial fluid from leaking out.
There are six basic categories of freely movable joints:
Rickets is a childhood disorder characterized by softening and weakening of the bones. Although the first scientific description of rickets came in the mid-1600s, it was not until about 1920 that the disease was associated with a deficiency of vitamin D. In the 1930s, the chemical structure of vitamin D was established-the vitamin is a fat-soluble steroid. Actually, the term vitamin D does not refer to a single molecule but rather to a family of related molecules. The active form is vitamin D3, although D3 itself can be converted in the body into other active molecules. In our discussions, we will use the generic term, vitamin D, to refer to the active substance.
Vitamins are generally defined as required substances that affect metabolic pathways and that the body cannot make, so they must be supplied in the diet. By this definition, vitamin D is technically not a vitamin. True, it is found in foods, particularly animal products, oily fish, and artificially fortified foods (such as milk, margarine, butter, and cereals). However, it can be produced in the body by the action of sunlight or ultraviolet light on a precursor molecule, 7-dehydrocholesterol, which is found in the skin of humans and most other higher animals.
This complex process is summarized in Figure 8. The vitamin D produced in skin enters the bloodstream. It is bound to proteins and transported to sites around the body. Although vitamin D affects numerous metabolic pathways, its primary role is to regulate the absorption and use of calcium and phosphorus in the body. In addition to increasing absorption of calcium and phosphorus in the intestines and kidneys, vitamin D helps maintain blood levels of these two important minerals. It also helps form strong bones by increasing the calcium content of bone. When there is a deficiency of vitamin D, calcium absorption decreases. In response, the body produces a hormone that removes calcium from bone in an attempt to raise blood-calcium levels. This results in weaker bone structure (rickets in children, osteomalacia in adults).