Fractures And Bone Repair 201

box 8.1. Clinical Correlations: Articular Cartilage and Joint Diseases 183

box 8.2. Clinical Correlations: Nutritional Factors in Bone Formation 199

box 8.3. Functional Considerations: Hormonal Regulation of Bone Growth 201

S? overview of bone

Bone is a connective tissue characterized by a mineralized extracellular matrix

Bone is a specialized form of connective tissue that, like other connective tissues, consists of cells and extracellular matrix. The feature that distinguishes bone from other connective tissues is the mineralization of its matrix, which produces an extremely hard tissue capable of providing support and protection. The mineral is calcium phosphate, in the form of hydroxy apatite aystals [Ca10(PO4)6(OH)2].

By virtue of its mineral content, bone also serves as a storage site for calcium and phosphate. Both calcium and phosphate can be mobilized from the bone matrix and taken up by the blood as needed to maintain appropriate levels throughout the body. Thus, in addition to support and protection, bone plays an important sec ondary role in the homeostatic regulation of blood calcium levels.

Bone matrix contains collagen type I and some collagen type V, along with glycosaminoglycans, glycoproteins, and sialoproteins

The major structural component of bone matrix is type I collagen and, to a lesser extent, type V collagen. These collagens constitute about 90% of the bone matrix. The matrix also contains ground substance in the form of glycosaminoglycans (hyaluronic acid, chondroitin sulfate, and keratan sulfate); small glycoproteins such as osteocalcin, osteonectin, and osteopontin; and several sialoproteins. The glycoproteins and sialoproteins of the ground substance play a role in binding calcium in the mineralization process. Both the collagen and the ground substance components become mineralized to form bone.

Within the bone matrix are spaces called lacunae (sing., lacuna), each of which contains a bone cell, or osteocyte. The osteocyte extends numerous processes into small tunnels called canaliculi. Canaliculi run through the mineralized matrix, connecting adjacent lacunae and allowing contact between the cell processes of neighboring osteo-cytes. In this manner, a continuous network of canaliculi and lacunae containing cells and their processes is formed throughout the entire mass of mineralized tissue. Electron micrographs show that osteocyte processes communicate by gap junctions. Bone tissue depends on the osteocytes, which are responsible for maintaining its viability.

In addition to the osteocyte, three other cell types are present in bone:

• Osteoprogenitor cells are cells that give rise to osteoblasts.

• Osteoblasts are cells that secrete the extracellular matrix of bone; once the cell is surrounded with its secreted matrix, it is referred to as an osteocyte.

• Osteoclasts are bone-resorbing cells present on bone surfaces where bone is being removed or remodeled (reorganized) or where bone has been damaged.

Osteoprogenitor cells and osteoblasts are developmental precursors of the osteocyte. Osteoclasts are phagocytotic cells derived from bone marrow. Each of these cells is described in more detail below.

s? bones and bone tissue

Bones are the organs of the skeletal system; bone tissue is the structural component of bones

Typically, a bone consists of bone tissue and other connective tissues, including hemopoietic tissue, fat tissue, blood vessels, and nerves. If the bone forms a freely movable (synovial) joint, hyaline cartilage is present. The ability of the bone to perform its skeletal function is due to the bone tissue and, where present, the hyaline or articular cartilage.

Bone tissue is classified as either compact (dense) or spongy (cancellous)

If a bone is cut, two distinct structural arrangements of bone tissue can be recognized (Fig. 8.1). A compact, dense layer forms the outside of the bone; a sponge-like mesh-work consisting of trabeculae (thin, anastomosing spicules of bone tissue) forms the interior of the bone. The spaces within the meshwork are continuous and, in a living bone, are occupied by marrow and blood vessels.

Bones are classified according to shape; the location of spongy and compact bone varies with bone shape

Spongy and compact bone tissues are located in specific parts of bones. It is useful, then, to outline briefly the kinds of bones and survey where the two kinds of bone tissue are located. On the basis of shape, bones can be classified into four groups:

• Long bones are longer in one dimension than other bones and consist of a shaft and two ends, e.g., the tibia and the metacarpals. A schematic diagram of a long bone sectioned longitudinally through the shaft is shown in Figure 8.2.

• Short bones are nearly equal in length and diameter, e.g., the carpal bones of the hand.

• Flat bones are thin and plate-like, e.g., the bones of the calvarium (skull cap) and the sternum. They consist of two layers of relatively thick compact bone with an intervening layer of spongy bone.

• Irregular bones have a shape that does not fit into any one of the three groups just described; the shape may be complex, e.g., a vertebra, or the bone may contain air spaces or sinuses, e.g., the ethmoid bone.

Long bones have a shaft, called the diaphysis, and two expanded ends, each called an epiphysis (see Fig. 8.2). The articular surface of the epiphysis is covered with hyaline cartilage. The flared portion of the bone between the diaphysis and the epiphysis is called the metaphysis. It extends from the diaphysis to the epiphyseal line. A large cavity filled with bone marrow, called the marrow or medullaiy cavity, forms the inner portion of the bone. In the shaft, almost the entire thickness of the bone tissue is compact; at most, only a small amount of spongy bone faces the marrow cavity. At the ends of the bone, the re-

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