VanPutte_AP_13e_Chap06_PPT_Accessible

Chapter 06: Skeletal System

Overview

• The skeletal system serves as the body's framework, maintaining shape and facilitating movement.

• Composed of bones, cartilage, and ligaments.

Functions of the Skeletal System

Basic Functions

• Support:

  • Bones provide a rigid framework.

  • Cartilage offers flexibility and strength (e.g., in the nose, ears, thoracic cage).

  • Ligaments connect bones to each other.

• Protection:

  • Skull protects the brain.

  • Ribs and sternum safeguard organs in the thoracic cavity.

  • Vertebrae encase the spinal cord.

• Movement:

  • Muscles are connected to bones via tendons.

  • Allow for movement while preventing excessive motion through ligaments.

• Storage:

  • Reservoir for calcium and phosphate, released as needed.

  • Adipose tissue is stored in marrow cavities.

• Blood Cell Production:

  • Bone marrow produces blood cells and platelets.

Cartilage

Types of Cartilage

• Hyaline Cartilage: Major type with a smooth matrix.

• Fibrocartilage: Provides tensile strength; found in intervertebral discs.

• Elastic Cartilage: Very flexible; found in the ear.

Cellular Structure

• Chondroblasts: Form the cartilage matrix.

• Chondrocytes: Mature cells within lacunae, maintain the matrix.

• Matrix Composition:

  • Collagen fibers for strength.

  • Proteoglycans for resiliency.

• Perichondrium: Double-layered sheath covering most cartilage.

  • Inner Layer: Contains chondroblasts, more delicate.

  • Outer Layer: Contains blood vessels and nerves.

Growth

• Appositional Growth: Chondroblasts form new matrix on the cartilage surface.

• Interstitial Growth: Chondrocytes divide and produce new matrix, expanding from within.

Bone Histology

Bone Matrix Composition

• Organic Component (35%): Collagen and proteoglycans.

• Inorganic Component (65%): Hydroxyapatite crystals (Ca10(PO4)6(OH)2).

  • Functional Impact:

    • Removal of minerals leads to bendable bones.

    • Removal of collagen makes bones brittle.

Bone Cells

Osteoblasts

• Responsible for bone formation via ossification.

• Produce collagen and release hydroxyapatite via exocytosis.

• Communicate through gap junctions.

Osteocytes

• Mature bone cells that maintain bone matrix.

• Found in lacunae and connected by canaliculi.

Osteoclasts

• Involved in bone resorption.

• Possess a ruffled border that interfaces with bone.

• Derived from monocytes.

Bone Structure Types

• Woven Bone: Randomly oriented collagen fibers; first type formed during ossification.

• Lamellar Bone: Mature and structured into sheets (lamellae) for strength.

Spongy Bone

• Composed of trabeculae that form a scaffold-like structure.

• Spaces filled with marrow, covered by endosteum.

• Oriented along lines of stress to provide support.

Compact Bone

Structure and Function

• Dense outer layer with fewer spaces than spongy bone.

• Composed of osteons (Haversian systems) characterized by:

  • Central canals containing blood vessels.

  • Concentric rings (lamellae) surrounding central canals.

Channels

• Perforating (Volkmann’s) Canals: Connect blood vessels from the periosteum to the osteocytes.

• Nutrient Transfer: Exchange occurs through canaliculi via interstitial fluid.

Bone Anatomy

Long Bone Structure

• Diaphysis: Shaft primarily made of compact bone.

• Medullary Cavity: Red marrow in children, yellow marrow in adults.

• Epiphysis: Ends of the bone made of spongy bone covered with compact bone and hyaline cartilage.

• Epiphyseal Plate: Growth plate that ossifies into the epiphyseal line after growth stops.

Periosteum

• Connective tissue covering the outer surface of bones.

• Composed of dense irregular collagenous tissue with blood vessels and nerves.

• Osteoblasts and osteoclasts present in the inner layer.

Endosteum

• Lines internal cavities of bones, consisting primarily of osteoblasts and osteoclasts.

Bone Development

Types of Ossification

• Intramembranous Ossification: Forms bones in flat bones (e.g., skull).

• Endochondral Ossification: Forms long bones from cartilage.

Development Processes

1. Intramembranous Ossification: Begins in embryonic mesenchyme; forms many skull bones.

2. Endochondral Ossification: Forms around week 4 of embryonic development and continues into young adulthood.

Bone Growth Phases

1. Cartilage model formation.

2. Bone collar formation when blood vessels invade.

3. Primary ossification center forms in the diaphysis.

4. Secondary ossification centers appear in the epiphyses.

5. Epiphyseal plate becomes an epiphyseal line after growth.

Bone Growth and Remodeling

Growth

• Length Increase: Occurs at the epiphyseal plate organized into specific zones.

• Width Increase: Involves periosteal ossification and osteoclastic activity in the endosteum.

Factors Affecting Bone Growth

• Nutrition (calcium, vitamin D, vitamin C).

• Hormones (growth hormone, thyroid hormone, sex hormones).

Bone Remodeling

Purpose and Process

• Converts woven bone to lamellar bone; adjusts shapes; repairs bones; regulates ions.

• Basic multicellular units (BMUs) comprising osteoclasts and osteoblasts remodel bones.

• Lifespan of a BMU is about 6 months, renewing the skeleton roughly every 10 years.

Impact of Mechanical Stress

• Increased stress enhances bone strength through remodeling; decreased stress results in more osteoclast activity.

Bone Fracture Classification

Classification Criteria

• Mechanism (traumatic vs. pathological), location, soft-tissue damage, fracture pattern, and number of fragments.

Fracture Types

• Comminuted: Multiple fragments.

• Spiral: Twisted fracture.

• Greenstick: Incomplete fracture common in children.

Bone Repair Stages

1. Hematoma Formation: Blood clot forms; tissue dies.

2. Callus Formation: Internal and external callus stabilize the fracture site.

3. Callus Ossification: Callus becomes spongy bone.

4. Remodeling: Spongy bone replaced by compact bone; site restored.

Calcium Homeostasis

Importance of Calcium

• Essential for muscle contraction, cardiac function, and cellular signaling.

Hormonal Regulation

• Parathyroid Hormone (PTH): Increases calcium levels by stimulating osteoclasts and kidney reabsorption.

• Calcitriol: Stimulates intestinal calcium absorption.

• Calcitonin: Lowers blood calcium levels by inhibiting osteoclasts.

Osteoporosis

Overview

• Loss of bone matrix leading to porous and weakened bones.

• More common in women due to loss of reproductive hormones and low calcium intake.

• Prevention and treatment options include hormone replacement therapy and dietary changes.

Effects of Aging

• Decreased bone matrix leads to brittleness and increased fracture risk.

• Osteoarthritis due to wear down of articular cartilage

• Symptoms can include loss of height and pain.