bones

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The Human Skeletal System

The skeletal system consists of various components that provide structure, protection, and movement for the body.

Key Components:

1. Bones (206 in adults): Provide support, protection, and movement.

2. Cartilage: Flexible connective tissue that cushions joints and supports structures.

3. Joints: Points where bones meet, allowing movement.

4. Ligaments: Connect bones to other bones, stabilizing joints.

5. Tendons: Connect muscles to bones, enabling movement.

6. Bone Marrow: Found inside bones; produces blood cells and stores fat.

Bones

Osseous tissue (bone tissue) is a specialized connective tissue consisting of:

• Cells: Osteoblasts, osteocytes, osteoclasts, and skeletal stem cells.

• Matrix:

  • Collagen fibers (Type I): Provides strength and flexibility.

  • Ground Substance: Includes hydroxyapatite crystals (calcium & phosphate) for rigidity.

Bone Classifications (Based on Shape)

Bone Structure

Each bone has two main tissue types:

1. Compact Bone (Cortical Bone)

   • Dense, hard, smooth.

   • Provides strength and protection.

   • Forms the outer layer of all bones.

2. Spongy Bone (Trabecular Bone)

   • Porous, lightweight, absorbs shock.

   • Contains bone marrow.

   • Found in epiphyses (ends of long bones) and inside flat & irregular bones.

Gross Anatomy of a Long Bone

Bone Cells & Their Functions

Bone Remodeling: Osteoblasts build new bone, while osteoclasts resorb old bone to maintain strength and mineral balance.

Cartilage

What is Cartilage?

• Strong yet flexible connective tissue.

• Avascular (no blood supply), heals slowly.

• Made of chondrocytes embedded in a matrix of collagen fibers & proteoglycans (trap water for shock absorption).

Types of Cartilage

Joints (Articulations)

Joints allow movement and provide stability.

Structural Classification (by Material)

Functional Classification (by Movement)

Bone Formation (Ossification)

Two Types of Bone Development

1. Intramembranous Ossification (Direct bone formation)

   • Occurs in: Flat bones (skull, clavicle)

   • Steps:

     1. Osteoblasts secrete osteoid (unmineralized bone).

     2. Calcium & phosphate deposit, hardening into bone.

     3. Trabeculae form, creating spongy bone.

     4. Outer compact bone layer develops.

2. Endochondral Ossification (Cartilage model replaced by bone)

   • Occurs in: Most bones (long bones, ribs, pelvis)

   • Steps:

     1. Cartilage model forms.

     2. Chondrocytes grow, cartilage calcifies.

     3. Osteoblasts replace cartilage with bone.

     4. Secondary ossification centers form at epiphyses.

     5. Growth continues at epiphyseal plate until puberty.

Bone Growth & Remodeling

1. Interstitial Growth (Lengthwise)

   • Occurs at the epiphyseal plate.

   • Chondrocytes divide, pushing old cells down where osteoblasts replace them with bone.

   • Stops after puberty when growth plate closes.

2. Appositional Growth (Widthwise)

   • Osteoblasts in periosteum deposit bone.

   • Osteoclasts inside medullary cavity resorb bone to maintain balance.

Bone Repair After a Fracture

1. Hematoma Formation – Blood clot stabilizes fracture.

2. Fibrocartilaginous Callus – Fibroblasts create soft callus.

3. Bony Callus Formation – Osteoblasts replace cartilage with bone.

4. Bone Remodeling – Osteoclasts reshape bone for strength.

Hormones Affecting Bone Growth

Aging & Bone Health

• Osteoporosis Risk: Bone resorption outpaces formation.

• Slower Healing: Reduced osteoblast activity.

• Decreased Bone Density: Trabecular bone is lost faster.

These detailed notes include everything you need for your Tuesday exam! Let me know if you need summaries, flashcards, or diagrams. 💀📚