I. Tissues and Organs of the Skeletal System (p. 231)

A. Functions of the Skeleton (p. 232)

1. The skeleton functions in support, protection, movement, blood formation, electrolyte balance, acid-base balance, and detoxification of the body. (table 8.1)

B. Bones and Osseous Tissue (p. 232)

1. The study of bones is osteology.

2. Osseous tissue is predominant in bones; also present are blood, marrow, cartilage, adipose tissue, nerves, and fibrous connective tissue.

C. The Shapes of Bones (p. 232; figs. 8.1, 8.2; TR 178–182)

1. Long bones include those in the appendages that produce body movement.

2. Short bones are equal in length and width, such as those of the wrist and ankle.

3. Flat bones, such as in the skull, protect soft tissues.

4. Irregular bones have elaborate shapes that don’t fit any of the previous categories. Example: the vertebrae.

D. General Features of Bones (p. 233)

1. The features of a long bone include its outer layer of compact bone, a medullary cavity containing bone marrow, and spongy bone at its ends.

2. The shaft of a long bone is referred to as the diaphysis; the expanded ends are the epiphyses.

3. The epiphyses are covered with articular cartilage, and the outer bone is covered by periosteum. The inside is lined with endosteum.

4. During growth, an epiphyseal plate of hyaline cartilage forms a model for bone to replace.

II. Histology of Osseous Tissue (p. 235; figs. 8.3, 8.4; TR 183)

A. Cells (p. 235)

1. Osteogenic cells develop from mesenchyme and occur in the endosteum, the inner periosteum, and the haversian canals. They are the only source of osteoblasts and osteocytes.

2. Osteoblasts are bone-forming cells that build new bone matrix.

3. Osteocytes are osteoblasts trapped in bone matrix. They remain active in bone maintenance.

4. Osteoclasts are bone-dissolving cells that form by fusion of monocytes. They break down bone and release its minerals to the blood.

B. Matrix (p. 235)

1. The organic matter in bone (one-third of the dry weight) is collagen, GAGs, proteoglycans, and glycoproteins.

2. The remainder is mineral components, especially hydroxyapatite and calcium carbonate. Other minerals are present in minute quantities.

C. Compact Bone (p. 236; fig. 8.5; TR 184)

1. Lamellae are arranged in concentric circles around haversian canals. This is the basic structural unit of compact bone, collectively called an osteon.

2. Within the lamellae lie the lacunae with osteocytes. Canaliculi extend between adjacent lamellae.

3. Perforating (Volkmann's) canals enter the bone from the outside and inside, and feed into the haversian canals, carrying nerves and blood vessels.

D. Spongy Bone (p. 237; fig. 8.6)

1. Spongy bone consists of slender rods, plates, and spines called trabeculae.

2. Bone marrow occupies the spaces within the trabeculae.

E. Bone Marrow (p. 239; fig. 8.7; TR 185)

1. In children, red marrow (myeloid tissue) is hemopoietic and fills the medullary cavity.

2. In young to middle-aged adults, most of the marrow in the medullary cavity is yellow marrow that stores fat.

3. In older adults, most of the yellow marrow is replaced by gelatinous marrow.

III. Bone Development (p. 240)

A. Intramembranous Ossification (p. 240; fig. 8.8)

1. Intramembranous ossification occurs within a membrane of soft tissue that represents the location of a future flat bone. Its cells differentiate into osteogenic cells and osteoblasts, and trabeculae are formed.

2. Osteoblasts form on the trabeculae and lay down an organic matrix and deposit calcium phosphate within it. When trapped, they become osteocytes.

B. Endochondral Ossification (p. 240; fig. 8.9; TR 186)

1. Endochondral ossification is bone formation using a cartilage model.

a. In the center of the model is the primary ossification center where lacunae enlarge and minerals are deposited around them.

b. Cells of the perichondrium become osteogenic cells and osteoblasts and produce bone on the outside of the model.

c. The primary ossification center becomes a primary marrow space.

2. The transitional zone between the head of hyaline cartilage and the shaft of a developing long bone is the metaphysis. (fig. 8.10)

a. The metaphysis exhibits five zones representing stages of ossification: the zone of reserve cartilage; the zone of cell proliferation; the zone of cell hypertrophy; the zone of calcification; and the zone of bone deposition. (fig. 8.11)

3. At birth, secondary ossification centers form in the epiphyses of long bones.

a. The epiphysis is hollowed out from the center outward and is replaced by bone.

b. The cartilaginous epiphyseal plates disappear by adulthood. (fig. 8.12)

C. Bone Growth and Remodeling (p. 243)

1. Bones grow in size and change in shape throughout life to accommodate the changing forces applied to the skeleton.

2. Bone development is a reflection of a person’s nutrition and physical exertion.

3. Cartilage can grow two ways: by interstitial growth (adding more matrix internally) and by appositional growth (adding more to the surface).

IV. Physiology of Osseous Tissue (p. 243)

A. Mineral Deposition (p. 244)

1. Mineralization (mineral deposition) is the process whereby calcium and phosphate are deposited in blood tissue.

2. Mineralization of bone is based on the action of seed crystals on an unstable solution of calcium and phosphate salts, in the presence of collagen fibers.

B. Mineral Resorption (p. 245)

1. Resorption is the process of dissolving bone to release its minerals to the bloodstream.

2. Osteoclasts dissolve bone using hydrochloric acid and acid phosphatase.

C. Calcium and Phosphate Homeostasis (p. 245)

1. The skeleton serves as a reservoir for calcium, phosphorus, and other minerals that play important roles in physiology.

2. Excessively low calcium concentration in the blood, called hypocalcemia, causes the nervous system to become hyperexcitable. Muscle tetany can result. (fig. 8.13; TR 188)

3. Excessive blood calcium, or hypercalcemia, can cause nervous system depression and sometimes cardiac arrest.

4. Calcium phosphate homeostasis is regulated by three hormones.

a. Calcitriol, an activated form of vitamin D, behaves like a hormone to influence bone deposition by stimulating the small intestine to absorb calcium and phosphate, reducing the urinary excretion of calcium and phosphate, and promoting osteoclast activity. (fig. 8.14; TR 189)

b. Calcitonin acts to lower blood levels of calcium by stimulating osteoblasts and inhibiting osteoclasts. (fig. 8.15a; TR 190)

c. Parathyroid hormone (PTH) raises blood calcium when it drops too low. PTH stimulates osteoclasts, lessens urinary excretion of calcium, and stimulates the synthesis of vitamin D. (fig. 8.15b; TR 190)

D. Other Factors Affecting Bone (p. 248)

1. At least 20 other hormones, growth factors, and vitamins affect osseous tissue in complex ways that are still not well understood. (table 8.2)

V. Bone Disorders (p. 248)

A. Fractures and Their Repair (p. 249; fig. 8.16; TR 191, 192; table 8.3)

1. The Healing of Fractures (fig. 8.17; TR 193)

a. A bone fracture results in a hematoma from torn blood vessels.

b. Next, soft granulation tissue forms as blood vessels grow into the hematoma. Macrophages remove debris as osteoclasts, osteogenic cells, and fibroblasts migrate into the area.

c. Fibroblasts deposit collagen, and a fibrocartilaginous callus is formed by chondroblasts. The callus is first soft, then becomes hard as it is replaced with bony tissue.

d. The area of the fracture is remodeled for 3 to 4 months until broken bone fragments are resorbed.

2. The Treatment of Fractures (p. 250)

a. Fractures may be set by closed reduction (no surgery) or by open reduction (surgical placement of bones, using pins and plates). (fig. 8.18)

b. Orthopedics is the branch of medicine dealing with injuries and disorders of bones, joints, and muscles.

B. Other Bone Disorders (p. 252)

1. The most common bone disease is osteoporosis.

2. Various bone disorders are summarized in table 8.4.