Anatomy & Physiology Skeletal System Notes
Functions of the Skeletal System
1. The skeletal system consists of bones, cartilage, tendons, and ligaments.
2. The skeletal system supports the body, protects organs it surrounds, allows body movement, stores minerals and fats, and is the site of blood cell production.
Cartilage
1. Chondroblasts produce cartilage and become chondrocytes. Chondrocytes are located in lacunae surrounded by matrix.
2. The matrix of collagen contains collagen fibers (for strength) and proteoglycans (trap water).
Bone Anatomy
A. Bone Shapes
1. Individual bones can be classified as long, short, flat, or irregular
B. Structure of a Long Bone
1. The diaphysis is the shaft of a long bone and the epiphyses are the ends.
2. The epiphyseal plate is the site of bone growth in length.
3. The medullary cavity is a space within the diaphysis.
4. Red marrow is the site of blood cell production and yellow marrow consists of fat.
5. The periosteum covers the outer surface of bone and consists of two layers.
· The outer layer contains blood vessels and nerves.
· The inner layer contains osteoblasts and osteoclasts.
· Perforating or Sharpey’s fibers hold the periosteum, ligaments and tendons in place.
6. The endosteum lines cavities inside bone.
C. Structure of Flat, Short, and Irregular Bones
1. Flat, short, and irregular bones have an outer covering of compact bone surrounding a layer of spongy bone.
Bone Histology
A. Bone Matrix
1. Bone matrix has two components, organic collagen fibers (35%) and an inorganic mineral material (65%) made up of mostly hydroxyapatite with chemical formula Ca10(PO4)6(OH)2. This is similar to reinforced concrete.
· Collagen like the reinforcing steel bars, provides flexible strength.
· Hydroxyapatite like the concrete, provides compressional or weight-bearing strength.
B. Bone Cells
1. Osteoblasts produce bone matrix and become osteocytes.
· Osteoblasts connect to one another through cell process and surround themselves with bone matrix to become mature bone cells or osteocytes.
· Osteocytes are located in lacunae and are connected to one another through canaliculi.
2. Osteoclasts break down bone.
C. Woven and Lamellar Bone
1. Woven bone has collagen fibers oriented in many different directions. It is remodeled with lamellar bone.
2. Lamellar bone is arranged in thin layers, called lamellae, which have collagen fibers oriented parallel to one another.
D. Spongy and Compact Bone
1. Spongy bone has many spaces.
· Lamellae combine to form trabeculae, beams of bone that interconnect to form a latticelike structure with spaces filled with bone marrow and blood vessels.
· The trabeculae are oriented along the lines of stress and provide structural strength.
2. Compact bone is dense with few spaces.
· Compact bone consists of organized lamellae – concentric circles of lamellae surround a central canal. Lacunae containing osteocytes are embedded in the lamellae. This is called a haversian system.
· Canals within compact bone provide a means for the exchange of gases, nutrients and waste products. From the periosteum or endosteum, perforating canals carry blood vessels to central canals. Canaliculi connect central canals to osteocytes.
Bone Development
A. Intramembranous Ossification
1. Some skull bones, part of the mandible, and the diaphyses of the clavicles develop from membranes.
2. Fontanels are areas of membranes that are not ossified at birth.
B. Endochondral Ossification
1. Most bones develop from a hyaline cartilage model.
2. Primary ossification centers form in the diaphysis during fetal development. Secondary ossification centers form in the epiphyses.
3. Articular cartilage on the ends of bones and the epiphyseal plate is cartilage that does not ossify.
Bone Growth
A. Growth at the Epiphyseal Plate
1. Epiphyseal plate growth results in an increase in length of the bone. This growth stops when the plate becomes the epiphyseal line.
2. Plate growth occurs first through cartilage cells dividing then bone growth on the cartilage.
B. Growth at Articular Cartilage
1. Growth under the articular cartilage results in an increase in the size of the epiphyses and growth in bones that do not have an epiphyseal plate.
C. Other Bone Growth
1. Bone growth under the periosteum increases the diameter of long bones and the size of other bones.
2. Bone resorption takes place in the medullary cavity of a long bone resulting in an increase in the size of the cavity and maintenance of the proportionate shape of the bone
D. Factors Affecting Bone Growth
1. Genetic factors affect the size and shape of bones but the expression of these factors can be modified.
2. Deficiencies in vitamin C or D affect collagen synthesis or mineralization of the matrix.
3. Hormones such as growth hormone, thyroid hormone, estrogen, and testosterone stimulate bone growth.
4. Estrogen and testosterone cause increased bone growth and closure of the epiphyseal plate.
Bone Remodeling
1. Remodeling converts woven bone to lamellar bone and allows bone to change shape, adjust to stress, repair, and regulate body calcium levels.
2. Bone adjusts to stress by adding new bone and by realignment of bone through remodeling.
Bone Repair
1. Fractures can be classified in many ways.
· Open, where the skin is broken versus closed, where the skin is not broken.
· Complete, where the bone is broken into at least two fragments versus incomplete where the fracture does not extend completely through the bone. Examples of incomplete fractures are hairline and greenstick. Greenstick fractures are common in children.
· Comminuted fractures are complete fractures in which bone breaks into more than two pieces.
· Impacted are those in which one fragment is driven into the spongy bone portion of the other fragment. This type of fracture is common in elderly or persons with osteoporosis.
2. Fracture repair begins with formation of a hematoma.
3. An internal callus consisting of fibrocartilage replaces the hematoma.
4. The external callus is a bone-cartilage collar that stabilizes the ends of the broken bone.
5. The internal and external calluses are ossified to become woven bone.
6. Woven bone is remodeled to lamellar bone. This process takes approximately one year.
Calcium Homeostasis
Parathyroid hormone or PTH stimulates osteoclast activity to increase bone breakdown and thus increases blood calcium levels. Calcitonin has the opposite effect.
Major Disorders of the Skeletal System
1. Arthritis is the inflammation of any joint and affects 10% of the world’s population.
· Osteoarthritis is the most common and is the degeneration of articular cartilage usually from normal “wear and tear”.
· Rheumatoid arthritis affects 3% of women and 1% of men in the US and is a severely disabling disease that results in deformation of the extremities.
2. Acromegaly, gigantism and dwarfism result from abnormal secretion of growth hormone.
3. Osteogenesis imperfecta results in brittle bones from insufficient collagen or mineralization of bones.
4. Osteoporosis is the reduction in the overall quantity of bone when bone resorption exceeds bone formation. Women can eventually lose half and men a quarter of their spongy bone. The spongy bone becomes so porous that it weakens and fractures easily. Osteoporosis increases with age and is 2.5 times more common in women than in men.
5. Bacterial infections and bone tumors can also occur in bone tissue.