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 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.
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 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
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).
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