Bone formation, growth, and remodeling

1. Bone formation, growth, and remodeling Pages 140-143

2. The skeleton is formed from two of the strongest and most supportive tissues in the body • Cartilage and bone • Embryos are made primarily of hyaline cartilage, over time the cartilage is replaced by bone • At birth, a newborn’s body has approximately 300 bones. Over time, these bones grow together to form the 206 bones in the adult body • Cartilage remains only in isolated areas such as the bridge of the nose, parts of the ribs, and joints

3. Osteoblasts vs. Osteoclasts • Osteoblasts (cells that help form bone) • used mainly in intramembranous ossification, are the specialized cells in bone tissue that deposit calcium into the protein matrix of bone (collagen) • Osteoclasts (cells that help eat away old bone) • used in endochondral ossification, dissolve calcium previously stored away in bone and carry it to tissues whenever needed.

4. Ossification • Ossification (bone formation) involves two major phases • First, the hyaline cartilage model is completely covered with bone matrix by osteoblasts (bone-forming cells) • Then the enclosed hyaline cartilage model is digested away by osteoclasts, opening up a medullary cavity within the newly formed bone. • By birth, most of the cartilage models have been converted to bone

5. Figure 21: Prenatal long bone development. Taken from:Stevens and Lowe, Human Histology, p. 246, Figure 13.24.

6. Appositional Growth • Growing bones must also widen as they lengthen • Osteoblasts in the periosteum add bone tissue to the external face of the diaphysis as osteoclasts in the endosteum remove bone from the inner face of the diaphysis wall • The two processes occur at the same rate so the circumference of the long bone expands and the bone widens • Growth hormones controls long-bone growth till puberty, then the sex hormones take over • Epiphyseal plates are then covered completely by bone

7. Bone is not dead! • Bones are remodeled continually in response to changes in two factors: • Calcium levels in the blood • The pull of gravity and muscles on the skeleton

8. Calcium level changes in the blood • When blood calcium levels drop below homeostatic levels, the paranthyroid glands in the throat are stimulated to release PTH (paranthyroid hormone) into the blood. • PTH activates osteoclasts to break down bone matrix and release calcium ions into the blood • Calcium is deposited in bone matrix as hard calcium salts when blood calcium levels are too high (Hypercalcemia)

9. Bone Remodeling • Bone remodeling is essential if bones are to retain normal proportions and strength during long-bone growth as the body increases in size and weight • The stresses of muscle pull and gravity acting on the skeleton determine where bone matrix is to be broken down or formed so that the skeleton can remain as strong and vital as possible • What happens to bedridden or physically inactive people’s bones?

10. Fractures • Fractures are breaks to the bone • During youth, most fractures results from exceptional trauma that twists or smashes the bones • In old age, bones thin and weaken, and fractures occur more often • Closed or simple fractures are clean breaks that do not penetrate the skin • Open or compound fractures are when the broken bone ends penetrate through the skin

11. Open/compound fractures are worse than Closed/simple fractures, why?

12. Stages of bone fracture healing • Fractures are treated by reduction which is the realignment of broken bone ends • Closed reduction—the bone ends are coaxed back into their normal position by the physician’s hands • Open reduction—Surgery is performed and the bone ends are secured together with pins or wires • Casts and tractions immobilize the broken bone to allow the healing process to begin • Simple fractures take about 6-8 weeks to heal • Takes longer for large bones and older people, why?

13. Four major events of bone fracture repair • 1. A blood-filled swelling, hematoma, forms to provide nutrition to bone cells where blood cells have ruptured • 2. Fibrocartilage callus (contains cartilage matrix, some bony matrix, and collagen fibers) act as a splint to the broken bone to close the gap • 3. The fibrocartilage callus is gradually replaced by one made of spongy bone, the bony callus

14. Four major events of bone fracture repair continued • 4. The bony callus is remodeled in response to the mechanical stresses on it, so that it forms a strong permanent “patch” at the fracture site.