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There are two major phases of ossification in long bones

1. Osteoblasts (builder cells) osteoblasts multiply (through mitosis) cartilage calcifies- it is replaced with bone by the

osteoblasts

2. Cartilage inside the diaphysis is digested away This opens up the medullary cavity

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By birth, most cartilage is converted to bone except:1. Articular cartilages (the epiphyseal surfaces)2. Epiphyseal plates

New cartilage is continuously formed by chondrocytes

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Bones grow in two ways:◦ length (longitudinal)◦ width (appositional)

Growth in diameter

Controlled by growth hormones Epiphyseal plates are converted from

cartilage to bone during adolescence ◦ Fused by the age of 18 (W), 21 (M)

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Bones are lengthened until growth stops

Bones are remodeled throughout life (every 7-10 years) in response to two factors:1. Blood calcium levels2. Pull of gravity and muscles on the skeleton

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Parathyroid hormone (PTH) ◦ Released when blood calcium levels are low◦ Activates osteoclasts (bone-destroying cells)

Osteoclasts break down bone and release calcium ions into the blood

Hypercalcemia (high blood calcium levels) prompts calcium storage to bones◦ Regulated by calcitonin (secreted by thyroid)

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Fracture: break in a bone Types of fractures

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Bone fractures are treated by reduction and immobilization

◦ Closed reduction: bones are manually coaxed into position by physician’s hands

◦ Open reduction: bones are secured with pins, screws, or wires during surgery

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1. Hematoma (blood-filled swelling) is formed2. Fibrocartilage callus forms

1. A soft mixture of cartilage matrix, bony matrix, and collagen fibers splint the broken bone

3. Bony callus (hard) replaces the fibrocartilage callus1. Osteoblasts and osteoclasts migrate in

4. Bone remodeling- compact bone replaces cartilage

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Figure 5.7 Stages in the healing of a bone fracture.

Hematoma

1

Externalcallus

Internalcallus(fibroustissue andcartilage)

New bloodvessels

Spongybonetrabecula

Hematomaforms.

2 3 Fibrocartilagecallus forms.

Bony callusforms.

Boneremodelingoccurs.

Bonycallus ofspongybone

Healedfracture

4

Common Types of Fractures• Closed (simple) fracture: break that does not penetrate the skin

• Open (compound) fracture: broken bone penetrates through the skin

• Comminuted: bone breaks into many fragments

• Compression: bone is crushed

• Depressed: broken bone portion is pressed inward

• Impacted: broken bone ends are forced into each other

• Spiral: ragged break occurs when excessive twisting forces are applied to a bone

• Greenstick: bone breaks incompletely (common in children)

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Table 5.2 Common Types of Fractures.