Skeletal system muscular system and integumentary system

Introduction

Adolescents, and many adults, take the health of their bone, muscle, and skin for granted. Only when there is a problem such as a broken bone, a muscle sprain, or a skin blemish (especially before an important event) do people think about these vital body systems. Health problems that affect bone, muscle, and skin are common. In fact, muscle and bone problems have prompted the World Health Organization to declare the years 2000–2010 the Bone and Joint Decade. Thirty-eight nations, including the United States, have endorsed this initiative.13, 14 The goals of the United States Bone and Joint Decade include an increased awareness of the burden of musculoskeletal disorders on society; the use of educational programs to promote prevention of these disorders; continuing research into the prevention, diagnosis, and treatment of musculoskeletal disorders; and improved treatment.

This section provides an introduction to the musculoskeletal and skin systems, including their involvement both in maintenance of good health and their dysfunction in disease. As such, it uses language and concepts not appropriate for middle school students. Our intention is to give you enough background that you will be able to assess your students' understanding of the topic and be equipped to answer their most common questions.

The musculoskeletal and skin systems and their functions are topics that are extremely well suited for middle school students. As stated in the National Science Education Standards (NSES), topics related to human biology are especially relevant to middle school students because students at this point in cognitive development begin to understand the relationship between structure and function.47 Middle school students are inherently interested in human biology because of the developmental changes they are experiencing. Students can integrate structure-function relationships in the context of human body systems working together.

Figure 1

Looking Good, Feeling Good: From the Inside Out addresses content and personal health standards from the National Science Education Standards.

In this module, students learn that their bones, muscles, and skin fulfill many roles that enable a person to complete complex voluntary tasks as well as involuntary actions that are essential to health. The information about the musculoskeletal and skin systems will also help students achieve the content standards related to Life Science, particularly concepts related to the structure and function of living systems. In addition, this module addresses standards related to Science in Personal and Social Perspectives (personal health). The concepts conveyed will also address several of the National Health Education Standards.

Misconceptions about the Musculoskeletal and Skin Systems

Adolescents, like many adults, have perceptions about their musculoskeletal and skin systems that are likely to be incorrect or incomplete. Almost every day, people are exposed to material on television or radio or in the newspapers about a new medicine, exercise, treatment, product, or diet that can influence their health. For example, advertisements promote "nutritional supplements" that will build muscle without exercise or dieting. Adolescents hear, see, or read about many over-the-counter treatments for acne—a condition about which they are especially aware because of their age. Many teenagers will try these products in search of help. Teenagers also receive inaccurate information about acne from peers or family members who believe that acne is caused by eating chocolate or other sweets. In addition, pharmaceutical companies often advertise prescription medications that are used to prevent disease. Although these medications can be valuable when used correctly, the advertisements do not give a complete picture.

Generally, science textbooks for middle school students present limited scientific information on the musculoskeletal and skin systems. As part of the presentation on the major body systems, science textbooks include a diagram of each of these systems with the parts labeled and some cursory information about their functions. Too often, however, this information becomes a vocabulary exercise without conveying any real understanding of how these systems work or regulate a vast array of physiological processes. Some misconceptions about the musculoskeletal and skin systems are the following:

Misconception 1: Muscles are only used for voluntary physical actions like walking, running, or throwing. Skeletal muscles are probably most familiar to middle school students even though other types of muscles, cardiac and smooth, are essential for life functions. The heart muscle is composed of a different type of muscle cell (cardiac muscle cells) and beats to move blood throughout the body. Smooth muscle cells line blood vessels and the intestinal tract to help move blood or food through those passages. The tongue is made up of muscle cells that enable us to speak and is also an important part of the digestive system.

Misconception 2: Your muscles turn to fat if you quit exercising. Misconception 2 is common not only among adolescents but also among adults and reflects a basic misunderstanding of how the body works. If a person stops exercising, his or her muscle cells may decrease in volume and become smaller. At the same time, a person may increase the volume of fat cells in his or her body. This concurrent change may give the impression that muscle is becoming fat, but this is not the case.

Fat cells are different from muscle cells; muscle cells do not turn into fat.

Misconception 3: Bones are not living structures. Adolescents may have conflicting ideas about whether bones are living structures, depending upon the context of the situation they are considering. On the one hand, they may believe that bones are just hard things that hold the body up and have muscles attached to them. On the other hand, teenagers recognize that broken bones heal. Few students have an understanding of how their bones grow during development or recognize that the bone marrow is critical for production of both red and white blood cells. Even maintenance of bone structure is a dynamic process; the action of specialized cells called osteoblasts to form new bone is counterbalanced by other cells, osteoclasts, which break down bone through resorption. As people age, bone resorption predominates over bone formation.15

Misconception 4: Diseases like osteoporosis or arthritis affect only old people, so teenagers do not need to be concerned about them. Although osteoporosis, a disease in which bone density decreases, affects older individuals, scientists now realize that it is important for young people to take care of their bones because this can influence the onset of osteoporosis in later life.29, 52 Exercise, including resistance and high-impact exercise, and good nutrition, including adequate calcium intake (1,300 milligrams per day for children ages 9 to 18), are important for optimal bone health.3, 41

Misconception 5: Acne is caused by eating chocolate or greasy foods. The exact cause of acne is not known. This incomplete understanding has allowed many myths about the causes of acne to become widespread. There is little evidence that diet causes or affects the course of acne. Rather, acne is caused by a number of interacting factors. One important factor is the increase in male hormones (androgens) that accompanies puberty in both boys and girls. Nearly 85 percent of adolescents and young adults develop acne. Since acne seems to run in families, it is thought to have a genetic component as well. Although the causes of acne are unclear, several factors have been shown to exacerbate the disorder. These include changing hormone levels in females (as before their menstrual periods), friction caused by rubbing the skin, irritants such as pollution, squeezing of the lesions, and vigorous scrubbing of the skin.43

Misconception 6: Body piercings and tattoos are completely safe. Body modifications involve breaking the skin, and consequently, carry a risk of infection. People with tattoos are nine times more likely to be infected with the hepatitis C virus than are people without tattoos.35 The American Red Cross prevents people from donating blood for one year after they get a tattoo, body piercing, or acupuncture treatments.9

Figure 2

Tattoos and body piercings involve breaking the skin and therefore carry a risk of infection.

There are health risks associated with body piercings and tattoos. Anyone considering undergoing these procedures should first research them, be aware of the health risks, find a provider who performs the procedure correctly, and use proper follow-up care.

Characteristics of Living and Nonliving Systems

It should be simple to distinguish between living and nonliving systems. After all, even children know that a rock is nonliving and a spider is a living creature. However, defining life is not a trivial task. Life has been defined in many ways for many different purposes, and there is no single definition that works for everyone. The Characteristics of Living Systems table lists some characteristics that are commonly found in definitions of living systems.

Characteristics of Living Systems

These characteristics were derived with the following in mind:

• they are simple and basic, with very few exceptions;

• they provide conditions sufficient to determine whether something is living or not;

• they are consistent with common usage and when applied to simple examples, allow those examples to be classified in the same way.

Some objects that are clearly nonliving are derived from once-living systems, however. A lump of coal is largely made up of material from plants that lived millions of years ago.

The ability to reproduce is often identified as a characteristic of living systems. This characteristic is not listed separately because bone, muscle, and skin are living systems, but they do not reproduce themselves. The cells of bone reproduce and carry out activities (such as making protein and depositing mineral) that allow bone to grow, repair, and remodel itself. Bone cells do not reproduce and make new bones. In the Characteristics of Living Systems table, reproduction falls under the characteristic "function according to a genetic blueprint."

Although a single-celled organism such as an amoeba is classified as living, the situation in multicellular organisms such as humans is more complicated. As with the amoeba, we can classify a human as living. Indeed, close examination of the human body reveals that it is composed of living cells, cells that were once living, and nonliving substances produced by living cells. These distinctions become important as we investigate the structures and functions of bone, muscle, and skin.

Characteristics of Bone, Muscle, and Skin

Human development is a complex process that begins with a fertilized egg cell and eventually gives rise to an adult human composed of over 100 trillion cells.30 As development proceeds, cells begin to take on specialized roles that remain stable throughout the life of the individual. Cells with the same function may group together in specific ways to form a colony of cells called a tissue. An adult human makes use of over 200 different tissues.60 One or more tissues may work together to form one of the body's organs. As the number of cells in the developing human increases, the fate of the individual cells becomes evermore restricted. This process by which a cell becomes committed to a specific function is called differentiation.

Just as the human body has different organs that carry out specific functions, the human cell has different organelles that have specialized functions. All human cells share certain characteristics. They

• possess a plasma membrane that separates their inside contents from the outside environment,

• enclose their genetic material inside a membrane-bound organelle called a nucleus,

• generate usable energy within organelles called mitochondria, and

• synthesize proteins using ribosomes.

Despite these similarities, differentiation produces cells that differ in significant ways from one another. The shapes of different cells relate to their functions within the body. For example, nerve cells have many long branches that enable them to communicate with each other and with other cells. Even the presence or absence of a critical organelle, such as the nucleus, can vary by cell type. A mature red blood cell has no nucleus, while a mature skeletal muscle cell has many nuclei derived from cells that have fused together. We shall learn in the following sections how the cells of the musculoskeletal and skin systems have characteristic shapes that relate to their functions and how they combine to form specialized tissues.

Bone

Bones serve many important functions. They allow us to do things we take for granted, such as stand and sit, walk and run. They do this in concert with muscles, which attach to bones via tendons. Our bones provide structural support for the body and help determine our shape. Bones also protect internal organs (the skull protects the brain, and the ribs protect the heart and lungs), and the bone marrow produces red blood cells and the white blood cells of the immune system. Bones are lightweight yet very strong, static in appearance yet very dynamic. How does the structure of bones determine how they function in the body?

Our skeletal system serves as a storage depot for calcium and other physiologically important ions.

Bones have a unique structure

The human skeleton has 206 bones of different sizes and shapes. Bones such as those in the arms and legs are called long bones. Others, such as those in the skull, are called flat bones. Other categories include the short bones (for instance, the carpel bones of wrist) and the irregular bones (for instance, vertebrae). In general, adult human bones are composed of about 70 percent minerals and 30 percent organic matter.20 The minerals are primarily a crystalline complex of calcium and phosphate called hydroxyapatite, while 90 to 95 percent of the organic matter is the protein collagen. The remainder of the organic matter consists of a gelatinous medium called ground substance, which contains extracellular fluid and specialized proteins called proteoglycans. How these organic and inorganic materials are put together to form the strong unit we call bone is discussed in section 4.1.3., Bone Formation.

Looking at a cross section of a long bone, one sees an inner cavity surrounded by an outer fibrous matrix. The inner cavity contains bone marrow, which consists of fatty tissue and cells that give rise to the red and white blood cells that circulate in the body. The bone matrix contains hydroxyapatite and calcium salts deposited in a network of collagen fibers. On the outside of the bone is a fibrous layer called the periosteum (Figure 3).

A closer look reveals more details. There are two forms of bone—compact (hard) bone, the solid, hard outside part of bone that is optimized to handle compressive and bending forces, and spongy (cancellous) bone, which is found inside the compact bone and near the ends of the bone. Blood vessels are also present and allow nutrients to be brought to bone cells and waste products to be carried away. Blood vessels and nerves pass through narrow openings, or canals, that run parallel to the surface and along the long axis of the bone.

Bone contains three specialized cell types

The name of each begins with osteo, since this is the Greek word for bone. Osteoblasts are cells that form new bone. They are found on the surface of new bone and they have a single nucleus (Figure 4). They are derived from stem cells in the bone marrow. Osteoblasts produce collagen found in bone and the proteoglycans found in ground substance. They are rich in alkaline phosphatase, a phosphate-splitting enzyme required for bone mineralization, a process that osteoblasts control. When osteoblasts have completed making new bone, the cells take on a flattened appearance and line the surface of the bone. Now in a more mature, less active state, the cells are called bone-lining cells. They still serve important functions, however. For instance, bone-lining cells respond to specific hormones and produce proteins that activate another type of bone cell called the osteoclast.

Figure 4

An osteoclast, osteoblasts, and osteocytes (across the bottom).

Osteoclasts are large, multinucleated cells that are capable of movement. They are formed by the fusion of mononuclear cells derived from stem cells in the bone marrow. Unlike osteoblasts, osteoclasts lie in depressions where their function is to dissolve (resorb) bone and help shape it (Figure 4). They begin by attacking the mineral portion of bone and then they degrade the bone proteins.

Osteocytes are cells that reside inside bone. They are derived from osteoblasts as new bone is being formed and then become surrounded by the new bone. However, rather than being isolated, osteocytes communicate through long branches that connect these cells to one another. These cells regulate the response of bone to its mechanical environment.53 Osteocytes sense pressures or cracks in bone and help direct osteoclasts to locations where bone will be remodeled.

Bone formation involves an organic matrix

To understand how bone is formed and why its properties confer such strength, imagine that you have steel rods and cement that you will use to construct a wall or a bridge. Pouring cement around steel rods placed in a suitable frame produces a material (reinforced concrete) that is stronger and more capable of withstanding movement than either steel rods or cement alone. Bone has a similar organization. The steel rods are chains of collagen, which confer resiliency, and the cement is hydroxyapatite, which confers strength.

Bone formation begins with synthesis of the organic matrix by osteoblasts. The matrix can be likened to a protein scaffolding. Next, through a mechanism not yet understood, osteoblasts deposit mineral crystals in the spaces between the protein scaffolding. The mineral consists primarily of calcium and phosphorus. Finally, osteoclasts work with osteocytes to shape or remodel the bone by breaking down the proteins and resorbing the minerals. Bone formation is not a strictly linear process, however. Bones are constantly being formed, broken down, and re-formed. Bone is a very dynamic, continually changing tissue. Osteoblasts, osteoclasts, and osteocytes function to maintain a balance between bone deposition and bone resorption that allows bones to grow, repair themselves, and remain strong.

The activity of osteoblasts and osteoclasts is influenced by a number of factors. Vitamin D helps the intestine absorb calcium from foods into the bloodstream after digestion. It is also important in regulating phosphate in the body (see also section 5.2, Vitamin D).23 Additionally, when blood-calcium levels are low, the parathyroid glands release parathyroid hormone into the blood. Parathyroid hormone activates the osteoclasts, thereby increasing the rate of bone breakdown. Other factors that regulate the dynamic balance between bone deposition and bone breakdown include growth factors and hormones. Importantly, exercise is an important factor in normal bone growth and development. Also, the composition of bone mineral is not fixed. Other ions, if present, can be incorporated into new or remodeled bone. Fluoride, for example, can be incorporated into bone mineral to form fluorapatite, which is harder, less soluble, and more resistant to resorption than is hydroxyapatite.

Bones grow as we grow

This is no surprise. In fact, more bone is formed during the first 20 to 30 years of life than is resorbed, resulting in an increase in bone mass. However, contrary to what some might think, long bones do not grow (or elongate) from the middle, a region called the diaphysis. Rather, the bones grow from their ends, regions called the epiphyses (singular is epiphysis).

Cartilage is a connective tissue specialized to handle mechanical stress without becoming distorted permanently. It is found in areas where shock-absorbing properties are needed or where smooth movement between bones (that is, at a joint) is required. As bones grow, additional cartilage is deposited at the epiphyseal, or growth, plate. This cartilage is the framework on which bone matrix is deposited. Bone growth continues as long as the growth plates are able to produce chondrocytes (cartilage-producing cells). The growth plate determines the length and shape of the mature bone and is the weakest part of the growing skeleton. The growth plate can be injured (fractured) during an acute incident, such as a fall, or from overuse, such as during intense sports training.2 If untreated, some growth plate fractures can lead to permanent damage and can cause bone growth to stop prematurely.44

Hormones are responsible for the cessation of growth. At the end of puberty, high levels of estrogen or testosterone cause the chondrocytes to die, and they are replaced by bone. It is during late adolescence that humans achieve their peak bone mass.33 Over the next 30 or more years, the human adult skeleton is maintained by precisely balanced bone formation and bone resorption.49 Sometime after humans reach their 60s, bone mass begins to decrease because new bone formation can no longer keep pace with bone resorption.

Adequate calcium intake during teen years, when bone formation is very active, is an important factor in preventing excessive bone resorption later in life.

Muscle

Muscle is the most abundant tissue in most animals. In vertebrates, such as humans, there are different types of muscle, and each has a unique cellular structure and function. Skeletal muscle enables us to walk, run, lift, or do other physical movements. It enables people to maintain their body posture. Skeletal muscle is also referred to as striated muscle because the arrangement of muscle fibers has a striped (striated) appearance when viewed under a microscope. Smooth muscle is found in the walls of the stomach and intestines, the urinary bladder, the bronchi of the lungs, and the arterial blood vessels. It functions to propel substances along their tracts within the body. Smooth muscle lacks striations and is composed of cells that are spindle shaped. A third type of muscle, cardiac muscle, makes up the heart and pumps blood throughout the body. As the name implies, skeletal muscle is intimately associated with the skeletal system, and for this reason, this module focuses on skeletal muscle and does not discuss cardiac and smooth muscle. Unless otherwise noted, the term muscle refers to skeletal muscle from this point on.

During human development, the differentiation of the muscle system is essentially complete just 8 weeks after fertilization. The first cells committed to form muscle in the developing embryo are called myoblasts. Some myoblasts divide rapidly, while others migrate to areas where muscle tissue needs to form, such as the developing limb buds. Once myoblasts arrive at their needed location, they stop cell division and begin to fuse together with adjoining myoblasts. The results of this cell fusion create a larger cell with many nuclei that share the same cytoplasm. These multinucleated cells continue to differentiate into a myotube, which is the basic structural cell of muscle tissue.

The most essential feature of muscle cells is their ability to generate force by contracting, or shortening—a function unlike that of other types of cells. In skeletal muscle, numerous myotubes bundle together to form a muscle. Within each myotube are thin and thick filaments. Under the microscope, the regular arrangement of these filaments accounts for the alternating light and dark bands seen in the tissue. The functional unit of the muscle is called the sarcomere. Each sarcomere has a dark Z line at each end. By examining the structure of the sarcomere, we can begin to appreciate how a muscle cell is able to contract and exert force on the skeletal system.

When researchers observed muscle contraction under the microscope, they noticed that the sarcomere shortened, that is, the Z lines moved closer together. This observation suggested that muscle contraction proceeds by having thin and thick filaments slide past each other, shortening the sarcomere.

This process is described by the sliding-filament model of muscle contraction (Figure 5). According to this model, the lengths of the thin and thick filaments do not change. Rather, the extent to which they overlap changes. As the amount of overlap between the thin and thick filaments increases, the length of the sarcomere decreases. Thin filaments are made of a protein called actin, and thick filaments are made of a protein called myosin. The myosin molecule has a long "tail" region with a protruding "head" at one end. The myosin head provides the energy needed to move the filaments past each other by breaking down the high-energy molecule ATP into ADP and inorganic phosphate.

Figure 5

The sliding-filament model of muscle contraction.

Muscle contraction is controlled by the nervous system. Nerves that interact with a muscle cell release a neurotransmitter, known as acetylcholine. This triggers electrical changes within the muscle cell that lead to the release of calcium ions from the sarcoplasmic reticulum (a specialized form of the endoplasmic reticulum). The calcium ions release an inhibitory mechanism and allow the actin and myosin filaments to slide past each other.

The muscle fibers themselves are not all identical. They can be classified as slow-twitch fibers or fast-twitch fibers. At Thanksgiving dinner, we refer to these different types of turkey muscle as dark meat and light meat. The dark meat is composed of muscle that has a large proportion of slow-twitch fibers. The slow-twitch fibers are made of muscle cells that have more mitochondria and therefore more red-colored cytochromes than cells from fast-twitch fibers. Slow-twitch fibers have less sarcoplasmic reticulum as compared with fast-twitch fibers. Slow-twitch fibers contract at a rate about five times longer than fast-twitch fibers. Fast-twitch fibers are specialized for generating rapid, forceful contractions for short-term activities such as jumping or sprinting over a period of a few seconds to about a minute. Some of our muscles, such as those controlling eye movements, are made almost exclusively from fast-twitch fibers. Slow-twitch fibers are specialized for prolonged activity over a period of minutes or hours. The soleus muscle in the lower leg is made up of slow-twitch fibers.

Most of our muscles are composed of a mixture of slow-twitch and fast-twitch fibers, and this mix varies among individuals. The ratio of slow-twitch to fast-twitch fibers for a given muscle is largely genetically determined, though some studies have shown that rigorous training can alter the ratio.10 This partly explains why some individuals excel at running sprints while others excel at running long distances.56

An important point to remember about muscle is that it only contracts and relaxes.

This means that in order to move a limb either up and down or back and forth, a pair of muscles must be involved. Indeed, skeletal muscles work in antagonistic pairs. For example, when a person bends his or her arm, the biceps contract (shorten) and the triceps relax (lengthen). When the arm straightens, the biceps relax and the triceps contract. Contraction is called the concentric phase, whereas the relaxation of the muscle is the eccentric phase. In general, most people think of muscles generating force only as they contract and get shorter. In the case of eccentric contractions, however, the muscles exert force even as they are lengthening. For example, to descend stairs in a controlled way, the quadriceps, or thigh muscle, must contract even as the movement of the knee joint tends to stretch it. Scientists are now recognizing that understanding more about eccentric contractions is important because they are common physiologically, are often associated with muscle soreness and injuries, and may be important in muscle-strengthening activities.56

Scientists continue to learn more about the value of regular exercise for maintaining or improving health. Exercise reduces the risk of certain medical conditions including heart disease and obesity and can help reduce complications in other diseases such as diabetes. Exercise is important for children and adolescents as well as for adults. Although in the past, weight (or resistance) training was not recommended for children, the American College of Sports Medicine recently advised that resistance training using nonmaximal weights and the supervision of a trained instructor is safe.8 In addition to helping build optimal bone mass and reducing the risk of obesity, youth resistance training may decrease the incidence of some sports injuries. The increase in muscular strength that occurs when an adolescent participates in resistance training appears to be a result of increased neuromuscular activation and coordination rather than muscle growth.19

Skin

Skin is the largest organ of the human body. Skin is in constant contact with the environment and plays several important roles in maintaining our health and well-being. It serves many purposes, including

• providing a barrier to microorganisms and toxins,

• preventing us from drying out,

• helping us to maintain temperature control,

• helping us sense pressure and temperature, and

• providing aesthetic and beauty qualities.

Skin is composed of distinct layers

In humans, a functional skin barrier is acquired by about 8.5 months of prenatal development. Babies born prematurely do not have an effective skin barrier and must be kept alive in sterile incubators until they develop the requisite protection. Skin has three layers—the epidermis, the dermis, and the subcutaneous fat layer. The thickness of the epidermis and dermis is different for skin with or without hair. Glabrous skin (skin without hair) has an epidermal layer that is about 1.5 millimeters (mm) thick and a dermal layer that is about 3 mm thick. Hairy skin has an epidermal layer that is 0.07 mm thick and a dermal layer that is 1 to 2 mm thick. The thickness of the subcutaneous fat layer varies throughout the body and from one individual to another (Figure 6).

Figure 6

Skin layers (epidermis, dermis, and subcutaneous fat layer). Not drawn to scale.

The outermost layer of skin, which we can see, is called the epidermis. The epidermis itself also has multiple layers. The outer layer of the epidermis consists largely of dead skin cells, which are being continuously sloughed off. In fact, most of the house dust that you see is actually composed of dead skin cells. This layer of skin does not feel pain because it lacks blood vessels and nerves. The living, multiplying skin cells are found at the bottom of the epidermis, the basal layer.21

Beneath the epidermis is the dermis, which provides a strong, resilient, and flexible infrastructure for the skin. The main component of the dermis is collagen, which accounts for nearly 70 to 75 percent of the skin's dry weight. Collagen is a versatile protein that provides strength.21, 28 It is necessary for healing wounds, but overproduction during healing leads to scars. Stretch marks are caused by collagen fibers that have been stretched to the point of tearing. Another important component of the dermis is elastin, which gives skin its elasticity.11 Collagen and elastin degenerate with age, causing wrinkles and sagging.

The dermis is supplied with nutrients and oxygen by blood vessels. A recent study suggests that blood vessels are not the only way skin cells get oxygen, however. According to Markus Stücker of Ruhr University, the atmosphere, thought to be unimportant, actually supplies the top 0.25 to 0.40 mm of skin with oxygen.54 This corresponds to the entire epidermis and a portion of the dermis below. This finding has implications for doctors treating skin diseases. Healthy skin that is cut off from the air can compensate by obtaining oxygen from the blood, while diseased skin appears unable to do this.

The blood vessels in the dermis hold as much as 25 percent of the body's blood supply at one time. Transdermal drugs take advantage of this vast network of blood vessels. Any substance that penetrates the epidermis and reaches the dermis can enter the bloodstream.11

The dermis is also rich in nerves. Sensations transmitted by nerves in the skin include touch, temperature, pain, itching, and pressure.28 Free nerve endings are scattered throughout the skin and are grouped around the bases of hair. They can register pain and pressure.21

The dermis contains hair follicles, sebaceous glands, and sweat glands. (Hair grows from the bulb at the hair follicle's base, which is in the subcutaneous fat layer.) On one side of the follicle is a sebaceous gland that produces an oily substance that lubricates the hair and epidermis. On the other side of the follicle is the erector pili muscle used to erect the hairs.

There are two types of sweat glands. The eccrine glands are located all over the skin surface. They produce a salty liquid that functions as a cooling mechanism when it evaporates from the skin surface. This liquid is somewhat acidic, which helps retard the growth of bacteria that live on the skin. The apocrine glands are thought to produce odors that serve as sexual messages. They are located under the armpits and on the genitals. Starting at puberty, these glands begin secreting a mixture of protein and fat. Bacteria can thrive in these environments and are responsible for body odor.

The bottom layer of the skin is the subcutaneous fat layer. This layer consists primarily of fat cells separated by bands of fibrous connective tissue. It provides a reservoir of energy as well as insulation and gives us our shape.21, 28 The subcutaneous fat layer may best be known because of cellulite. Cellulite is the puckered appearance of skin thought to be caused by fibrous bands dividing lobules of fat.11, 28

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.

Figure 7

Clear skin is a sign of general good health and one aspect of sexual attraction.

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.

Interactions

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.

Joints

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 vertebra 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:

• Ball-and-socket joints allow more freedom of movement than any other joint. One bone of the joint has a rounded head that fits into the socket of the other bone. Ball-and-socket joints are found at the hips and shoulders.

• Gliding joints are composed of bones that are almost flat and slide over one another. They offer flexibility in movement direction, although they do not allow great range in movement. The bones of the wrist are connected through gliding joints.

• Hinge joints allow movement up and down, although they do not allow twisting or sliding. One bone, the humerus (bone of the upper arm), for example, fits into the rounded part of another bone, the ulna (the fixed bone of the forearm), thus forming a hinge joint (the elbow).

• Pivot joints allow bones to spin around one another. They are formed when one bone fits into the ring shape of another bone. A pivot joint allows the twisting motion of the elbow (as opposed to the up-and-down motion of the hinge joint). Additionally, a pivot joint is found between the first two vertebrae in the neck, allowing us to provide a negative response by rotating our head left and right.

• Condyloid joints allow movement in many directions, although they do not allow rotation. One bone of the joint is concave while the other is convex. The lower jaw is attached to the skull through a condyloid joint.

• Saddle joints allow side-to-side movement and limited rotation. The bones of a saddle joint have odd shapes but are completely complementary to one another. The lower finger bones are connected to the bones of the hand through saddle joints.

Vitamin D

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).

Figure 8

Vitamin D production. (Not drawn to scale.)

Vitamin D deficiency may also affect muscles, since calcium is essential for normal nerve impulse transmission and muscle contraction. Calcium deficiency may result in prolonged muscle spasms and muscle pain.

Exposure to the sun is a double-edged sword. On the one hand, excess exposure has adverse effects on the skin and is associated with increased risks of skin cancer. On the other hand, insufficient exposure to the sun may lead to vitamin D deficiency and its attendant bone and muscle problems. It is possible, however, to both protect against sun-damaged skin and get sufficient exposure to ensure adequate vitamin D synthesis. For example, in summer it takes only 10 to 15 minutes of sunlight on the face and wrists, two to three times a week between 8 a.m. and 4 p.m. to allow the body to make enough vitamin D. Sunscreen, which can block the beneficial effects of sun exposure, should be used to protect the skin at all other times.

Other factors influence vitamin D synthesis, including air pollution, which blocks sunlight; dark pigmentation in the skin; and clothing worn to cover skin. Additionally, vitamin D synthesis may decrease during the winter in geographic locations where the days are shorter, more time is spent indoors, and people dress warmly and cover up well when outdoors.23, 24

Diseases and Disorders

There are many diseases and disorders of bone, muscle, and skin. However, it is beyond the scope of this section to discuss them all. We provide here brief introductions to diseases and disorders related to lessons in this module and to several that students may ask about.

Diseases affecting bone

Achondroplasia

Achondroplasia is a genetic disorder of bone growth that affects males and females of all races and occurs in about 1 of every 25,000 births. People with achondroplasia exhibit abnormal body proportions; their arms and legs are very short while their torso is nearly normal in size. In normally proportioned people, cartilage develops into bone during fetal development and childhood. This process is abnormally slow in the growth plates of long bones in people with achondroplasia, resulting in shorter bones in the arms and legs.

Fractures

A bone can fracture, or break, when the pressure on it becomes too great, and the fracture can occur in several different ways. For instance, the bone may break across its width, lengthwise, at an angle, or spirally. The bone may crack rather than break all the way through or, in the case of extreme force, the bone may actually shatter. The broken bone may not damage surrounding tissue or penetrate the skin. On the other hand, it may damage the skin and surrounding tissue and increase the risk of infection to both the skin and the bone. Additionally, fractures may produce significant blood loss since bones have a rich blood supply.

The underlying causes of fractures are varied. Most common are fractures that result from injury. The injury may be acute, such as a fall, or it may occur more slowly over time, such as that resulting from a repetitive activity (for instance, running) that aggravates a susceptible site. Additionally, diseases (such as osteoporosis and cancer), nutritional deficiencies (such as of calcium and vitamin D), and some medications can make bone more susceptible to fracture.

Osteoarthritis

Osteoarthritis, the most common type of arthritis, affects the cartilage that covers the ends of bones in a joint. In osteoarthritis, also called degenerative joint disease, the cartilage breaks down and causes pain, swelling, and loss of motion. In addition, bone spurs may form and bits of bone or cartilage can break off and float inside the joint space.45

Scientists do not yet know what causes osteoarthritis. Like most diseases, scientists believe that genetic factors are an important contributing factor. Physicians can treat individuals with osteoarthritis using a number of strategies, including pain management, exercise, rest and joint care, weight control, medications, and nontraditional treatment approaches. Individuals who are concerned about the degeneration of joint cartilage sometimes take various nutritional supplements, most notably chondroitin and glucosamine (natural components of cartilage), to help maintain healthy cartilage and thereby reduce the risk or severity of osteoarthritis. Scientific evidence for the efficacy of these products varies.

Osteoporosis

Osteoporosis is a disease in which bone loss occurs accompanied by a decrease in bone strength. Women are four times more likely to have osteoporosis than men. Three factors contribute to osteoporosis:

• accelerated bone loss at menopause in women or as a consequence of aging in both men and women,

• failure to reach peak bone mass during childhood and adolescence, and

• bone loss that results from other conditions, such as eating disorders or certain medical treatments.41

Scientists now believe that the best time for preventing osteoporosis may be during childhood and adolescence, when bones are growing rapidly. In healthy children, adequate intake of calcium and vitamin D and weight-bearing physical activity may be sufficient to prevent osteoporosis.29

Figure 9

Healthy behaviors early in life can help prevent bone disorders such as osteoporosis later in life.

Treatments for osteoporosis are aimed at altering the imbalance between decreased bone formation and increased resorption.

Some medications inhibit the formation or activity of osteoclasts.46 Scientists are also investigating therapies that would stimulate bone formation by osteoblasts.51

Osteogenesis imperfecta

Osteogenesis imperfecta is also known as brittle bone disease. It affects an estimated 20,000 to 50,000 individuals in the United States.57 The news media will periodically present a case study of someone with this disease. Because it is usually diagnosed in children, it has, at times, been attributed incorrectly to child abuse. This inherited genetic disease is characterized by bones that break easily. In this disease, collagen is either formed in inadequate amounts or is changed structurally.48

There is no cure for this disease. Treatments focus on preventing or controlling the symptoms, maximizing mobility, and developing optimal bone mass and muscle strength.48 The goal for treatment is to prevent fractures and deformities while allowing the child with the disease to function as independently as possible.

Diseases and injuries affecting muscle

The ability of muscles to function effectively can be impaired by a number of diseases and injuries. In some diseases, the muscle tissue may not be the primary organ affected but may be impaired as a consequence of a problem at another site in the body.

Amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is also called Lou Gehrig's disease, and it has received media coverage because of the fame of the baseball player, Lou Gehrig. More recently, the book Tuesdays with Morrie (by Mitch Albom, 1997) told the story of a college professor who was afflicted with the disease. Individuals with ALS lose control of voluntary muscles because the nerves that innervate them are destroyed. As the disease progresses, muscles continue to weaken until they become paralyzed. There is currently no treatment for ALS. Most afflicted people die of respiratory failure because ALS paralyzes the muscles used for breathing.34

Muscular dystrophy

Muscular dystrophy is a group of genetic diseases characterized by progressive muscle weakness and loss of muscle tissue. Diagnosis of a specific form of muscular dystrophy is based on the individual's symptoms, the age of onset, and the nature of the genetic transmission. The most common types of muscular dystrophy are due to a deficiency in the muscle protein dystrophin.36

Duchenne's muscular dystrophy is the most severe form of the disease. Initially, it affects the muscles of the pelvis, upper arms, and upper legs. Becker's muscular dystrophy is milder than Duchenne's and progresses more slowly. Both Duchenne's and Becker's muscular dystrophy afflict boys almost exclusively.37 There is currently no cure for any type of muscular dystrophy. Treatments may include physical therapy, medications, assistive devices, and surgery.

Sprains and strains

Ankle sprains are the most common injury, often occurring during sports or recreational activities.40 A sprain occurs when a ligament, the tissue that connects bones at a joint, stretches or tears. A strain is an injury to either muscle or tendon. The immediate treatment for these injuries is "RICE" (Rest, Ice, Compression, Elevation).

Tetany

Tetany is condition characterized by muscle spasms (twitching and cramps) brought on when calcium levels in body fluids fall below normal. This can be associated with calcium or vitamin deficiencies, the pathogenic bacterium Clostridium tetani, or with other medical conditions. When calcium levels are below normal, the nervous system becomes more excitable and nerves fire spontaneously. These impulses cause skeletal muscles to contract spasmodically. Tetany can usually be treated with calcium, vitamin D, and diet.

Because skin diseases are so visible to others, affected individuals may feel embarrassed or even ashamed about their conditions.

Disorders of skin

The psychological effects of skin conditions can be more serious than the conditions themselves. Young children with skin diseases may be subject to teasing, while teenagers, concerned about their physical appearance, may be shunned by their peers. Adults, too, feel the stigma associated with skin disease. They may become more withdrawn and even refuse to seek treatment for their condition, believing (often erroneously) that nothing can be done for them. A recent survey indicated that less than half of the people with a skin disorder seek advice from a doctor, but instead rely on others, such as pharmacists, for information.32 It is likely that there are significant numbers of people who are misinformed about their conditions and may be relying on treatments that have little value.

Some of the more common skin diseases include

• acne,

• atopic dermatitis (eczema),

• skin cancer,

• alopecia (hair loss), and

• vitiligo.

Many of these diseases can affect individuals of all races and both genders. For example, acne is a problem for both males and females of all races. Other diseases, however, are more prevalent in some populations than others. For example in the United States, scleroderma has a higher prevalence among certain Native Americans, and keloids occur more frequently in African-American individuals.42

The following discussion focuses on those disorders with particular relevance to young people and those relevant to this module.

Acne

Acne is a skin disorder that results when hormones interact with the skin's sebaceous glands. These glands produce an oily substance called sebum. Sebum normally reaches the skin surface through an opening of a hair follicle called a pore. During acne, the pore becomes blocked by the hair, sebum, and the cells that line the follicle. This mixture of oil and cells allows the bacteria that normally live on the skin to grow within the plugged pore. Substances produced by the bacteria attract white blood cells and cause inflammation, characterized by redness, swelling, heat, and pain. Eventually, the wall of the follicle breaks down and its contents spill into the surrounding skin, producing a lesion commonly called a pimple. People with acne usually exhibit a variety of skin lesions that may include whiteheads and blackheads. Whiteheads are plugged follicles that remain below the skin surface and produce a white bump. If a plugged follicle reaches the skin surface, it appears black and is called a blackhead.

Figure 10

. Acne is a common skin disease that can be effectively treated.

Treatments for acne include oral and topical medications used alone or in combination. Most of these medications retard or halt bacterial growth and reduce inflammation. Severe acne can be treated effectively using isotretinoin (Accutane). This drug usually clears up acne in 15 to 20 weeks and also helps prevent scarring. It can, however, cause birth defects in the developing fetus of a pregnant woman. Therefore, women who take the drug and are of child-bearing age must use two forms of birth control before, during, and after treatment. Another possible side effect of taking Accutane is mental disorders. Some patients taking Accutane have become depressed or developed other serious mental problems. More recently, isotretinoin has been shown to produce detrimental effects on bone.17

Alopecia

Alopecia refers to the partial or complete loss of hair, and it may result from genetic factors, normal aging, certain medications, or disease. Male-pattern alopecia is very common. Although the condition is genetic and related to the male sex hormones, its precise cause is unknown. Female-pattern alopecia is also common, although it is usually characterized by thinning of the hair in specific regions of the head rather than complete hair loss as in males. Alopecia areata is a disorder affecting 4 million Americans, 60 percent of them under the age of 20. It causes sudden hair loss on the scalp and other body regions because follicles stop producing hair; bald patches can appear overnight. The precise cause of this disorder is unknown, although it is thought to be an autoimmune disorder. While alopecia areata is not life threatening, and while it can disappear as quickly as it appeared, it can be difficult for children to cope with psychologically.

Atopic dermatitis

Atopic dermatitis, often called eczema, is a chronic skin disease that most often afflicts infants and young children, although it can arise in adults as well. Major symptoms of atopic dermatitis include intense itching, rashes in areas characteristic of the disease, repeatedly occurring symptoms, and a personal or family history of atopic disorders such as hay fever, asthma, and eczema. Symptoms can be made worse by exposure to irritants such as wool or synthetic fibers, poorly fitting clothes that chafe the skin, soaps that dry out the skin, and allergens (substances in plants or animals that trigger an immune reaction).

The cause of atopic dermatitis is not known. Evidence suggests that it is a multifactorial disease, meaning that both genetic and environmental factors interact to produce the symptoms. Although stress is known to make the condition worse, it is not a cause of the disorder. Atopic dermatitis is not infectious and cannot be passed from one person to another. The disorder seems to be associated with hay fever and asthma. Many children who outgrow atopic dermatitis later develop hay fever and asthma.

Treatment of atopic dermatitis involves healing the skin and keeping it healthy, preventing the onset of symptoms, and treating the symptoms when they arise. An important aspect of treatment is daily skin care, which involves proper bathing and application of lubricants immediately after bathing. When symptoms occur, they may be treated using corticosteroid creams and ointments. Phototherapy with ultraviolet light, sometimes in conjunction with drugs called psoralens, may control the symptoms. Adults with severe forms of the disease may be treated with immunosuppressive drugs such as cyclosporine.

Skin cancer

Skin cancer (including melanoma and nonmelanoma skin cancer) is the most common cancer, accounting for more than 40 percent of all cancers. About 1.3 million cases of nonmelanoma skin cancer are diagnosed each year in the United States.5 Skin cancer is a disease in which cells of the epidermis lose their ability to control their own growth. These cancer cells form tumors that, if left untreated, can spread to other parts of the body and produce serious, often fatal disease.

Cancer can arise in any of the three types of cells found in the epidermis: round cells called basal cells; flat, scaly surface cells called squamous cells; and melanocytes, which give skin its characteristic color. Basal cell cancer is the most common form. It is usually found in body areas that have been exposed to the sun. It may appear as a small, raised bump with a smooth texture. Another form resembles a scar and is firm to the touch. Basal cell cancers may spread to other tissues around the cancer, but they usually do not spread to other parts of the body. Squamous cell tumors also appear in areas that have been exposed to the sun. They may also appear in areas that have been burned, exposed to chemicals, or subjected to X-ray therapy. Squamous cell tumors often appear as firm, red bumps. They may feel scaly, bleed, or develop a crust. Squamous cell tumors can spread to lymph nodes in the area.

A more serious form of skin cancer is called melanoma (cancer in the melanocytes). Melanoma accounts for about 4 percent of skin cancer cases but causes approximately 79 percent of skin cancer deaths.6 It usually occurs in adults and may appear as a new mole or as a change in an existing mole. Melanoma can spread cancer to other parts of the body. Early diagnosis of melanoma is critical for effective treatment.

Sun exposure is linked to more than 90 percent of nonmelanoma skin cancers.39

Exposure to the sun's radiation has been identified as the primary risk factor for skin cancer. Shorter-wavelength ultraviolet light called UVB rays cause tanning and burning in humans. A 1 percent increase in UVB-ray exposure may produce a 2 percent increase in skin cancer incidence. Longer wavelength UVA rays penetrate more deeply into the dermis. UVA rays produce an immediate darkening of the skin as it is absorbed by melanin. Exposure to UVA rays can weaken the skin's inner connective tissue, damage the immune system, and possibly lead to cancer.1 UVA rays can also burn the eyes and result in cataracts. Severe, blistering sunburns, particularly those experienced during childhood or as a teenager, increase an individual's risk of developing melanoma.7, 38 Although proponents of tanning beds argue that they are safe, the mere fact that they expose the skin to ultraviolet radiation means that they contribute to the risk of skin cancer.

Rates of skin cancer are also influenced by the degree of skin pigmentation. Rates are highest for those with the lightest-colored skin. Theoretically, all skin cancer is preventable. It has been estimated that most people get about 80 percent of their lifetime sun exposure before the age of 18. Therefore, it is prudent to protect children from needless sun exposure. Those at greatest risk for skin cancer (children and those with light skin) should use protective clothing and sunscreens whenever possible.

Figure 11

UV light used in tanning beds carries similar health risks to sunlight.

Skin cancers are treated by several different methods, depending on the type of cell involved and to what extent the cancer has progressed. The most common approach is to surgically remove the tumor by cutting, burning, or freezing. Another technique is radiation therapy, which uses X-rays to kill cancer cells and shrink tumors. Chemotherapy uses drugs to kill cancer cells and may be applied topically to the skin or given systemically, in which case it enters the bloodstream and can kill cancer cells in other areas of the body. Biological therapies are now being developed that use the body's immune system to fight the disease.

Vitiligo

Vitiligo is a skin condition resulting from a loss of pigment. Any part of the body may be affected and develop the characteristic white patches of the disorder. Vitiligo affects 1 to 2 percent of the population, and about half of those affected first exhibit the disorder before the age of 20. There appears to be a genetic component to vitiligo, and it is thought to be an autoimmune disorder affecting the pigment-producing cells of the skin. Although the disease can be managed with medications and other treatments, there is no cure for it.

Influences

The structure and function of bone, muscle, and skin are subject to many influences. Some influences as well as some of their negative ramifications are presented in the following table.

Bone, Muscle, and Skin Can Be Influenced by Many Things

The factor that has the greatest influence on these body systems is genetics. We have no control over this factor, nor do many of us have control over our exposure to environmental pollutants. However, the key for middle school students is to realize that we do have control over many of the large number of factors that influence our bone, muscle, and skin, through the choices we make. For instance, we choose not only how long we will expose ourselves to the sun, but also what protective measures we will use, such as sunscreens and hats. Within cultural, social, and economic limits, we make decisions about the foods we eat and thus determine our intake of key nutrients, such as protein and calcium. We are influenced by peers and the media and make decisions about risky behaviors, such as smoking. We make decisions about exercise, about what we slap (sometimes slop) on our skin, about what we can do to enhance our appearance—all generally done thinking only about now and without concern for any long-term effects on our bone, muscle, and skin.

This curriculum supplement introduces students to three key body systems. It also provides an opportunity to introduce students to decision making and to realizing that decisions made today have consequences in the future.

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Glossary

A neurotransmitter whose release leads to muscle contraction.

A contractile protein found in muscle cells. Together with myosin, actin provides the mechanism for muscle contraction.

A large sweat gland found in hairy regions of the body. Starting at puberty, apocrine glands secrete a mixture of protein and fat. Bacteria can thrive in these environments and are responsible for body odor. The glands are also thought to send sexual messages through odor. Modified apocrine glands produce mother's milk.

Small, round cells located in the lower portion of the skin's epidermis.

A soft tissue found in the center of large bones. Bone marrow produces blood cells.

A chemical element that plays a vital role in the biochemistry of a cell. Calcium is an important part of a healthy diet. It is stored in the skeleton and released into the bloodstream as needed.

An elastic connective tissue. Unlike bone, cartilage does not contain blood vessels and lacks the ability to regenerate.

A type of cell found in cartilage.

The primary protein found in connective tissue. It gives skin its elasticity and provides strength to ligaments and tendons.

The layer of skin found beneath the epidermis. It contains many nerve endings that are responsible for the sense of touch.

The shaft of a long bone.

During development, the process by which individual cells in the body take on specialized functions.

A sweat gland that participates in regulating the temperature of the body. It is found in the skin over most parts of the body.

The outermost layer of the skin.

The end portions of long bones where growth occurs.

A fluorine-containing mineral that contributes strength to bone.

Material in which cells of connective tissue are embedded. It is also called matrix.

An infolding of the epidermis that contains the root of an individual hair.

A mineral containing calcium and phosphorous that contributes strength to bone.

A location where bones meet and allow movement about that location.

An overgrowth of fibrous scar tissue caused by excessive tissue repair at the site of a skin injury.

The primary structural protein found in hair and nails.

Cells of the epidermis that synthesize keratin.

Connective tissue that connects bone to bone.

A dark pigment produced in the bottom layer of the skin's epidermis. Melanin absorbs ultraviolet light and is largely responsible for the color of the skin.

A type of cell found in the epidermis that produces the pigment melanin.

A form of skin cancer that derives from a melanocyte.

An immature muscle cell.

A contractile protein found in muscle cells. Together with actin, myosin provides the mechanism for muscle contraction.

A mature muscle cell.

A part of the body that performs a specific function. The heart and lungs are examples of organs.

A cell that contributes to the formation of bone.

A cell that contributes to the breakdown and resorption of bone.

A branched cell found in the bone matrix. Osteocytes are derived from osteoblasts. They communicate with other cells and help the bone respond to its environment.

A class of proteins with a high polysaccharide content.

The structural and functional unit of muscle contraction.

The form of endoplasmic reticulum found in muscle fibers.

A chronic disorder characterized by hardening and thickening of the skin.

A gland that secretes an oily substance into a hair follicle to lubricate hair or skin.

A rule stated as, "short shadow—seek shade." It is a convenient way to assess when protection from the sun is needed.

A model that describes how muscle cells contract using filaments made of actin and myosin.

A layer of flat skin cells found in the epidermis.

A layer of fat cells beneath the dermis.

Small tubular glands found nearly everywhere in the skin that secrete perspiration through pores in the skin.

a fluid found in joints that provides lubrication.

A band of connective tissue that attaches muscle to bone.

One of two types of filaments found in myofibrils. It is mainly composed of the protein myosin.

One of two types of filaments found in myofibrils. It is mainly composed of the protein actin.

A population of similar cells that act together to perform one or more specific functions in the body.

A portion of the electromagnetic spectrum having wavelengths shorter than visible light but longer than X-rays.

Long wavelength (320–380 nanometers) ultraviolet light.

Short wavelength (280–320 nanometers) ultraviolet light.

A fat-soluble vitamin needed for normal growth of bone. Vitamin D is produced when sterols in the body are irradiated by ultraviolet light.