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.
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.
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
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.
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.
|Composed of one or more cells|
|Function according to a genetic blueprint|
|Obtain and generate energy (that is, have a metabolism)|
|Interact with their environment|
These characteristics were derived with the following in mind:
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.
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
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.
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 under Bone formation involves an organic matrix.
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.
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.