Review of Key Concepts
Tissues and Organs of the Skeletal
System (p. 218)
- Osteology is the study of bone. The skeletal system consists of bones, cartilages, and ligaments.
- The functions of the skeletal system include bodily support, protection of internal organs, movement, blood formation, electrolyte and acid-base balance, and detoxification.
- Bone (osseous tissue) is a connective tissue with a mineralized matrix.
- Bones are classified by shape as long, short, flat, and irregular bones.
- A long bone is a cylinder of compact bone enclosing more porous spongy bone and a central medullary cavity.
- The shaft of a long bone is the diaphysis and each expanded end is an epiphysis. The epiphysis is covered with articular cartilage where it meets an adjacent bone. An epiphyseal plate separates the medullary cavities of the diaphysis and epiphysis in children and adolescents.
- A bone is externally covered with a fibrous periosteum and internally lined with a thin endosteum.
Histology of Osseous Tissue (p. 221)
- Osseous tissue has four types of cells: stem cells called osteogenic cells, matrix-depositing cells called osteoblasts, strain detectors called osteocytes enclosed in lacunae of the matrix, and matrix-dissolving cells called osteoclasts.
- The bone matrix is a composite of collagen fibers and other protein-carbohydrate complexes embedded in a ground substance of hydroxyapatite, calcium carbonate, and other minerals. The minerals enable bones to resist compression and support weight, and the proteins enable bones to bend slightly without breaking.
- Compact bone consists mostly of osteons. An osteon is a cylindrical group of concentric lamellae of matrix surrounding a central canal occupied by a nerve and blood vessels. Interstitial lamellae, the remains of old osteons, occupy some of the spaces between osteons, and circumferential lamellae parallel to the bone surface form the inner and outer boundaries of compact bone. Blood vessels reach the central canals by way of perforating canals that open on the bone surface.
- Spongy bone is a porous lattice of bony trabeculae with lamellae but few osteons. The trabeculae are oriented along lines of stress, giving spongy bone strength without excessive weight.
- The medullary cavity and the spaces between spongy bone trabeculae are occupied by bone marrow. Red bone marrow is hemopoietic (blood-forming) tissue; yellow bone marrow is adipose tissue; and gelatinous bone marrow is a reddish jelly found in older people.
Bone Development (p. 225)
- Intramembranous ossification is a process in which flat bones develop from sheets of embryonic mesenchyme. Osteoblasts deposit an organic matrix that transforms the mesenchyme into a soft collagenous osteoid tissue.
- Osteoblasts then deposit calcium salts that harden the osteoid tissue and transform it to spongy bone. The surface zones of this spongy bone are gradually filled in with calcified matrix to become compact bone.
- Endochondral ossification is a process in which bones develop from hyaline cartilage. Most of the fetal skeleton forms in this manner.
- Endochondral ossification begins with enlargement of the lacunae and death of chondrocytes in the primary ossification center, which is soon hollowed out to form a primary marrow space. Osteoblasts populate this space and create a temporary scaffold of spongy bone.
- Between the epiphysis and the primary marrow space is a metaphysis, which exhibits five zones of histological transformation from cartilage to bone.
- Secondary ossification centers appear later in the epiphyses of a long bone. The cartilaginous epiphyseal plate persists through adolescence as a growth zone for the long bone.
- Mature bones continue to be remodeled throughout life, employing appositional growth to enlarge and reshape bone surfaces in response to stress.
Physiology of Osseous Tissue (p. 229)
- The addition of inorganic salts to osseous tissue is called mineral deposition. Osteoblasts produce collagen fibers on which hydroxyapatite and other minerals crystallize. Mineralization requires a critical ratio of calcium to inorganic phosphate called the solubility product.
- The removal of salts from osseous tissue is called mineral resorption. Osteoclasts resorb bone by secreting hydrochloric acid, which dissolves the inorganic salts of the matrix, and acid phosphatase, an enzyme that digests the organic matrix.
- The skeleton is the body's major reservoir of calcium, which is also needed for many other physiological processes. The Ca2+ concentration of the blood plasma is maintained within narrow limits by resorbing Ca2+ from the skeleton or depositing excess Ca2+ into it.
- Hypocalcemia, a Ca2+ deficiency, can cause potentially fatal muscle tetany, while hypercalcemia, a Ca2+ excess, can depress neuromuscular function. These imbalances are normally prevented by the actions of calcitriol, calcitonin, and parathyroid hormone.
- Calcitriol is a form of vitamin D synthesized by the sequential action of the skin, liver, and kidneys. It raises blood Ca2+ concentration by promoting absorption of Ca2+ by the small intestine, conservation of Ca2+
Saladin: Anatomy & 7. Bone Tissue Text © The McGraw-Hill
Physiology: The Unity of Companies, 2003 Form and Function, Third Edition by the kidneys, and resorption of Ca2+ from bone by osteoclasts.
- Calcitonin is secreted by the thyroid gland in response to hypercalcemia. It lowers blood Ca2+ by inhibiting osteoclasts and stimulating the depositional activity of osteoblasts.
- Parathyroid hormone is secreted by the parathyroid glands in response to hypocalcemia. It raises blood Ca2+ by indirectly stimulating osteoclasts, inhibiting osteoblasts, promoting calcitriol synthesis, and promoting Ca2+ conservation by the kidneys.
- Many other hormones, growth factors, and vitamins influence bone physiology, such as growth hormone, testosterone, thyroid hormone, estrogen, insulin, and vitamins A and C.
Bone Disorders (p. 234)
1. The repair of a fractured bone begins when fibroblasts, osteogenic cells, and other cells invade the blood clot (fracture hematoma) and transform it into granulation tissue. Collagen and cartilage deposition convert this to a soft callus, which is then mineralized to form a hard callus. The hard callus is remodeled by osteoclasts and osteoblasts over several months.
Chapter 7 Bone Tissue 241
- Closed reduction is the realignment of the parts of a broken bone without surgery. Open reduction involves surgery and fragment realignment with the aid of plates and screws.
- The most common bone disease is osteoporosis, a loss of bone mass resulting in brittleness and abnormal vulnerability to fractures. It is most common in postmenopausal white women. The risk of osteoporosis can be minimized with diet (adequate calcium intake) and exercise.
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