PhatoExam3 Bone- Types of bone tissue

Chapter 9 Bone- Types of bone tissue Bone tissue consists of two types, which differ in density. Bone- Types of bone tissue is formed when many Haversian systems are tightly packed together, producing a very strong, rigid structure that forms the outer covering of bones. Cancellous or spongy bone is less dense and forms the interior structure of bones. Spongy bone lacks Haversian systems but is made up of plates of bonebordering cavities that contain marrow. A typical long bone consists of the diaphysis, a thin shaft, between two larger ends or epiphyses (see Fig. 9-1B). The diaphysis is formed of compact bone surrounding a medullary cavity containing marrow. The metaphysis is the area where the shaft broadens into the epiphysis. The epiphysis is made up of spongy bone covered by compact bone. The end of each epiphysis is covered by hyaline cartilage (articular cartilage), which facilitates movement at points of articulation between bones. The epiphyseal cartilage or plate (“growth” plate) is the site of longitudinal bone growth in children and adolescents, such growth being promoted by growth hormone and sex hormones. Longitudinal bone growth ceases when the epiphyseal plate ossifies during adolescence or early adulthood depending on the specific bone. The epiphyseal plate is referred to as the epiphyseal line following ossification or closure. No bone growth in length occurs after the epiphyseal plate becomes the epiphyseal line. However, bone may change in density or thickness at any time under the influence of hormones such as growth hormone, parathyroid hormone, or cortisol. The stress (weight-bearing or muscle tension) placed on the bone also affects the balance between osteoblastic and osteoclastic activity. With aging, bone loss is accentuated, resulting in decreased bone mass and density. Osteoporosis, loss of bone density due to loss of calcium salts, is common in older people, particularly women (see Chapter 24). Except for the surface of the bone covered by articular cartilage, the bone is covered by periosteum, a fibrous connective tissue. The periosteum contains osteoblasts, blood vessels, nerves, and lymphatics, some of which penetrate into the canals in the bone. When the periosteum is stretched or torn, severe pain results. The medullary cavity is lined with endosteum, also containing osteoblasts. These osteoblasts are required for bone repair and remodeling as needed. At birth the medullary cavity in most bones contains red bone marrow in which hematopoiesis takes place. Gradually, yellow (fatty) bonemarrow replaces red bone marrow in the long bones. In adults, red bone marrow is found in the cranium, bodies of the vertebrae, ribs, sternum, and ilia, the last two being the usual sites of bonemarrow aspiration used in the diagnosis and monitoring of leukemias and blood dyscrasias. Joints Joints or articulations between bones vary in the degree of movement allowed: •Synarthroses, represented by the sutures in the skull, are immovable joints. •Amphiarthroses, slightly movable joints, are joints in which the bones are connected by fibrocartilage or hyaline cartilage. Examples of this type of joint include the junction of the ribs and sternum and the symphysis pubis. •Diarthroses or synovial joints are freely movable joints and are the most common type of joint in the body. Different types of diarthroses allow a variety of movements. For example, a hinge joint, providing flexion and extension, is found at the elbow, whereas a ball-and- socket joint at the shoulder provides a wide range of motion, including rotation. Both hinge and gliding movements are found in the temporomandibular joint (TMJ), controlling the opening of the mouth. Common body movements are illustrated in Ready Reference 1 (see Fig. RR1-6). In a synovial joint, the ends of the bone are covered with articular (hyaline) cartilage, providing a smooth surface and a slight cushion during movement (see Fig. 9-13A). With aging, the cartilage in joints tends to degenerate and become thin, leading to difficulty with movement and potential changes in the alignment of bones. The joint cavity or space between the articulating ends of the bones is filled with a small amount of synovial fluid, which facilitates movement. The synovial fluid prevents the articular cartilage on the two surfaces from damaging each other and also provides nutrients to the articular cartilage. The synovial fluid is produced by the synovial membrane (synovium), which lines the joint capsule to the edge of the articular cartilages. The synovial membrane is well supplied with blood vessels. The articular capsule is composed of the synovial membrane and its outer covering, the fibrous capsule, a tough protective material that extends into the periosteum of each articulating bone (Sharpey's fibers). The capsule is reinforced by ligaments, straps across the joint that link the two bones, which support the joint and prevent excessive movement of the bones. In a few joints there are some variations in structure. The knee has additional moon-shaped fibrocartilage pads, termed lateral and medial menisci, which act to stabilize the joint. Bursae are fluid-filled sacs composed of synovial membrane and located between structures such as tendons and ligaments; they act as additional cushions in the joint. The TMJ, the only movable joint in the skull and face, has two synovial cavities and a central articular cartilage of dense collagen tissue. 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