SKELETAL SYSTEM INTRODUCTION A bone may appear to be inert because of nonliving material in the extracellular matrix of bone tissue. However, bone also includes active, living tissues: bone tissue, cartilage, dense connective tissue, blood, and nervous tissue. Bones are not only alive, but also multifunctional. Bones, the organs of the skeletal system, support and protect softer tissues, provide points of attachment for muscles, house blood-producing cells, and store inorganic salts. Skeletal system performs several basic functions: 1. Support: The skeleton serves as the structural framework for the body by supporting soft tissues and providing attachment points for the tendons of most skeletal muscles. 2. Protection: The skeleton protects the most important internal organs from injury. For example, cranial bones protect the brain, vertebrae (backbones) protect the spinal cord, and the rib cage protects the heart and lungs. 3. Assistance in movement: Most skeletal muscles attach to bones; when they contract, they pull on bones to produce movement. 4. Mineral homeostasis (storage and release): Bone tissue stores several minerals, especially calcium and phosphorus, which contribute to the strength of bone. Bone tissue stores about 99% of the body’s calcium. On demand, bone releases minerals into the blood to maintain critical mineral balances (homeostasis) and to distribute the minerals to other parts of the body. 5. Blood cell production: Within certain bones, a connective tissue called red bone marrow produces red blood cells, white blood cells, and platelets, a process called hemopoiesis. Red bone marrow consists of developing blood cells, adipocytes, fibroblasts, and macrophages within a network of reticular fibers. It is present in developing bones of the fetus and in some adult bones, such as the hip bones, ribs, breastbone, vertebrae (backbones), skull, and ends of the bones of the arm and thigh. 6. Triglyceride storage: Yellow bone marrow consists mainly of adipose cells, which store triglycerides. The stored triglycerides are a potential chemical energy reserve. In a newborn, all bone marrow is red and is involved in hemopoiesis. With increasing age, much of the bone marrow changes from red to yellow. Bone Structure: The bones of the skeletal system vary greatly in size and shape. However, bones are similar in structure, development, and function. Bone Classification Bones are classified according to their shapes long, short, flat, or irregular. Long bones have long longitudinal axes and expanded ends. Examples of long bones are the forearm and thigh bones. Short bones are cubelike, with roughly equal lengths and widths. The bones of the wrists and ankles are this type. Flat bones are platelike structures with broad surfaces, such as the ribs, scapulae, and some bones of the skull. Irregular bones have a variety of shapes and are usually connected to several other bones. Irregular bones include the vertebrae that comprise the backbone and many facial bones. A typical long bone consists of the following parts: 1. The diaphysis is the bone’s shaft or body—the long, cylindrical, main portion of the bone. 2. The epiphyses (singular is epiphysis) are the proximal and distal ends of the bone. 3. The metaphyses (singular is metaphysis) are the regions between the diaphysis and the epiphyses. In a growing bone, each metaphysis contains an epiphyseal (growth) plate, a layer of hyaline cartilage that allows the diaphysis of the bone to grow in length. When a bone ceases to grow in length at about ages 18–21, the cartilage in the epiphyseal plate is replaced by bone; the resulting bony structure is known as the epiphyseal line. 4. The articular cartilage is a thin layer of hyaline cartilage covering the part of the epiphysis where the bone forms an articulation (joint) with another bone. Articular cartilage reduces friction and absorbs shock at freely movable joints. 5. The periosteum surrounds the external bone surface wherever it is not covered by articular cartilage. It is composed of an outer fibrous layer of dense irregular connective tissue and an inner osteogenic layer that consists of cells. Some of the cells of the periosteum enable bone to grow in thickness, but not in length. 6. The medullary cavity or marrow cavity is a hollow, cylindrical space within the diaphysis that contains fatty yellow bone marrow in adults. 7. The endosteum is a thin membrane that lines the internal bone surface facing the medullary cavity. It contains a single layer of cells and a small amount of connective tissue. Composition: Four types of cells are present in bone tissue: osteogenic cells, osteoblasts, osteocytes, and osteoclasts. 1. Osteogenic cells are unspecialized stem cells derived from mesenchyme, the tissue from which almost all connective tissues are formed. They are the only bone cells to undergo cell division; the resulting cells develop into osteoblasts. Osteogenic cells are found along the inner portion of the periosteum, in the endosteum, and in the canals within bone that contain blood vessels. 2. Osteoblasts (blasts =buds or sprouts) are bone-building cells. They synthesize and secrete collagen fibers and other organic components needed to build the extracellular matrix of bone tissue, and they initiate calcification. As osteoblasts surround themselves with extracellular matrix, they become trapped in their secretions and become osteocytes. (Note: The ending -blast in the name of a bone cell or any other connective tissue cell means that the cell secretes extracellular matrix.) 3. Osteocytes (cytes = cells), mature bone cells, are the main cells in bone tissue and maintain its daily metabolism, such as the exchange of nutrients and wastes with the blood. Like osteoblasts, osteocytes do not undergo cell division. 4. Osteoclasts (clast = break) are huge cells derived from the fusion of as many as 50 monocytes and are concentrated in the endosteum. The osteoclast’s plasma membrane is deeply folded into a ruffled border. Here the cell releases powerful lysosomal enzymes and acids that digest the protein and mineral components of the underlying bone matrix. This breakdown of bone extracellular matrix, termed resorption, is part of the normal development, maintenance, and repair of bone. In response to certain hormones, osteoclasts help regulate blood calcium level. They are also target cells for drug therapy used to treat osteoporosis. The extracellular matrix is about 25% water, 25% collagen fibers, and 50% crystallized mineral salts. The most abundant mineral salt is calcium phosphate. It combines with another mineral salt, calcium hydroxide, to form crystals of hydroxyapatite. As the crystals form, they combine with still other mineral salts, such as calcium carbonate, and ions such as magnesium, fluoride, potassium, and sulfate. As these mineral salts are deposited in the framework formed by the collagen fibers of the extracellular matrix, they crystallize and the tissue hardens. This process, called calcification, is initiated by bone-building cells called osteoblasts. Bone is not completely solid but has many small spaces between its cells and extracellular matrix components. Some spaces serve as channels for blood vessels that supply bone cells with nutrients. Other spaces act as storage areas for red bone marrow. Depending on the size and distribution of the spaces, the regions of a bone may be categorized as compact or spongy. Overall, about 80% of the skeleton is compact bone and 20% is spongy bone. Factors Affecting Bone Growth: Normal bone metabolism; growth in the young depends on several factors. These include adequate dietary intake of minerals and vitamins, as well as sufficient levels of several hormones. 1. Minerals: Large amounts of calcium and phosphorus are needed while bones are growing, as are smaller amounts of magnesium, fluoride, and manganese. These minerals are also necessary during bone remodeling. 2. Vitamins: Vitamin A stimulates activity of osteoblasts. Vitamin C is needed for synthesis of collagen, Vitamin D helps build bone by increasing the absorption of calcium from foods in the gastrointestinal tract into the blood. Vitamins K and B12 are also needed for synthesis of bone proteins. 3. Hormones: During childhood, the hormones most important to bone growth are the insulin like growth factors (IGFs), which are produced by the liver and bone tissue. IGFs stimulate osteoblasts, promote cell division at the epiphyseal plate and in the periosteum, and enhance synthesis of the proteins needed to build new bone. IGFs are produced in response to the secretion of human growth hormone (hGH) from the anterior lobe of the pituitary gland. Thyroid hormones (T3 and T4) from the thyroid gland also promote bone growth by stimulating osteoblasts. In addition, the hormone insulin from the pancreas promotes bone growth by increasing the synthesis of bone proteins. At puberty, the secretion of hormones known as sex hormones causes a dramatic effect on bone growth. The sex hormones include estrogens (produced by the ovaries) and androgens such as testosterone (produced by the testes). Although females have much higher levels of estrogens and males have higher levels of androgens, females also have low levels of androgens, and males have low levels of estrogens. These hormones are responsible for increased osteoblast activity and synthesis of bone extracellular matrix. Divisions of the Skeletal System The adult human skeleton consists of 206 named bones, most of which are paired, with one member of each pair on the right and left sides of the body. Bones of the adult skeleton are grouped into two principal divisions: the axial skeleton (80 bones) and the appendicular skeleton (126 bones). The axial skeleton consists of the bones that lie around the longitudinal axis of the human body, skull bones, auditory ossicles (ear bones), hyoid bone, ribs, sternum, and bones of the vertebral column. Functionally, the auditory ossicles in the middle ear, which vibrate in response to sound waves that strike the eardrum, are not part of either the axial or appendicular skeleton, but they are grouped with the axial skeleton for convenience.