Structure of Bone and Cartilage Coverings

Layers and Cell Types

The coverings of bone and cartilage are specialized structures that provide protection, nourishment, and support for growth and repair.

• Periosteum: A dense, fibrous membrane covering the outer surface of bones, containing osteoblasts (bone-forming cells) and osteoclasts (bone-resorbing cells).

• Endosteum: A thin membrane lining the inner surfaces of bones, including the medullary cavity.

• Perichondrium: A layer surrounding cartilage, containing chondroblasts (cartilage-forming cells).

• Cell Types: Osteoblasts, osteoclasts, osteocytes (mature bone cells); chondroblasts, chondrocytes (cartilage cells).

Cartilage Growth

Interstitial vs. Appositional Growth

Cartilage grows by two mechanisms:

• Interstitial Growth: Chondrocytes divide and secrete new matrix from within the cartilage.

• Appositional Growth: New layers of cartilage are added to the surface by chondroblasts in the perichondrium.

Bone Marrow

Types and Functions

Bone marrow is a soft tissue found in the cavities of bones.

• Red Marrow: Site of hematopoiesis (blood cell formation).

• Yellow Marrow: Contains adipocytes (fat cells); serves as energy storage.

Types of Bone

Spongy vs. Compact Bone

Bones are classified based on their structure:

• Spongy (Cancellous) Bone: Porous, contains trabeculae; found at the ends of long bones and inside flat bones.

• Compact Bone: Dense, forms the outer layer of bones; composed of osteons (Haversian systems).

Vascularization of Bone

Bones are highly vascularized, receiving blood from nutrient arteries, periosteal vessels, and metaphyseal arteries.

Bone Cells

Osteoblasts, Osteoclasts, Osteocytes

• Osteoblasts: Synthesize bone matrix and promote mineralization.

• Osteoclasts: Resorb bone by secreting acids and enzymes.

• Osteocytes: Mature bone cells maintaining bone tissue.

Ossification

Endochondral vs. Intramembranous Ossification

Bone formation occurs by two processes:

• Endochondral Ossification: Bone develops from a cartilage template; typical of long bones.

• Intramembranous Ossification: Bone forms directly from mesenchymal tissue; typical of flat bones (e.g., skull).

Type of Cartilage in Endochondral Ossification

• Hyaline Cartilage: Serves as the template for endochondral bone formation.

Primary vs. Secondary Ossification Centers

• Primary Center: Appears in the diaphysis (shaft) during fetal development.

• Secondary Center: Appears in the epiphyses (ends) after birth.

Role of Periosteal Bud

The periosteal bud brings blood vessels, osteogenic cells, and nerves into the cartilage model, initiating bone formation.

Bone Growth

Appositional vs. Interstitial Growth

• Appositional Growth: Increases bone diameter by adding new bone to the surface.

• Interstitial Growth: Increases bone length via activity at the epiphyseal plate.

Functional Zones of the Growth (Epiphyseal) Plate

The epiphyseal plate is responsible for longitudinal bone growth and consists of several zones:

• Resting Zone

• Proliferation Zone

• Hypertrophic Zone

• Calcification Zone

• Ossification Zone

Hormones Affecting Bone Growth

• Growth Hormone: Stimulates overall bone growth.

• Thyroid Hormone: Regulates bone growth and development.

• Sex Hormones (Estrogen, Testosterone): Promote growth spurt and epiphyseal plate closure.

Substances Used by Osteoclasts

• Acids: Dissolve mineral components of bone.

• Enzymes: Break down organic matrix.

Wolff's Law

Bone adapts to the mechanical stresses placed upon it, becoming stronger with increased load and weaker with disuse.

Bone and Calcium Homeostasis

Role of Calcitonin & Parathyroid Hormone, Feedback Mechanism

• Calcitonin: Lowers blood calcium by inhibiting osteoclast activity.

• Parathyroid Hormone (PTH): Raises blood calcium by stimulating osteoclasts and increasing calcium reabsorption in kidneys.

Feedback Mechanism: Blood calcium levels are regulated by negative feedback involving calcitonin and PTH.

Fracture Classification and Common Types

• Simple (Closed) Fracture: Bone breaks but does not penetrate skin.

• Compound (Open) Fracture: Bone breaks and penetrates skin.

• Comminuted Fracture: Bone shatters into multiple pieces.

• Greenstick Fracture: Incomplete break, common in children.

• Spiral Fracture: Caused by twisting forces.

Fracture Healing

Stages of Healing

• Hematoma Formation: Blood clot forms at fracture site.

• Fibrocartilaginous Callus Formation: Soft callus bridges the fracture.

• Bony Callus Formation: Hard callus replaces soft callus.

• Bone Remodeling: Bone is reshaped to original form.

Osteoporosis vs. Osteomalacia

• Osteoporosis: Decreased bone mass and density, increased fracture risk.

• Osteomalacia: Softening of bones due to defective mineralization (often from vitamin D deficiency).

Joint Structure and Movement Classifications

• Fibrous Joints: Bones joined by dense connective tissue; little or no movement (e.g., sutures).

• Cartilaginous Joints: Bones joined by cartilage; limited movement.

• Synovial Joints: Freely movable joints with a synovial cavity.

Cartilaginous Joint Types

• Synchondrosis: Bones joined by hyaline cartilage (e.g., epiphyseal plate).

• Symphysis: Bones joined by fibrocartilage (e.g., pubic symphysis).

Function of Articular Cartilage

Articular cartilage covers the ends of bones in synovial joints, reducing friction and absorbing shock.

Synovial Joint Structure and Features

• Articular Cartilage: Covers bone surfaces.

• Joint (Synovial) Cavity: Space filled with synovial fluid.

• Synovial Membrane: Produces synovial fluid.

• Fibrous Capsule: Encloses joint cavity.

• Ligaments: Strengthen and stabilize joints.

Shoulder Joint Features

• Ball-and-Socket Joint: Allows greatest range of motion.

• Glenoid Cavity: Shallow socket for humeral head.

• Rotator Cuff Muscles: Stabilize the joint.

Sprain vs. Dislocation

• Sprain: Stretching or tearing of ligaments.

• Dislocation: Displacement of bone from its joint.

Composition of Connective Tissue Matrices

• Fibers: Collagen, elastic, reticular.

• Ground Substance: Proteoglycans, glycosaminoglycans.

• Cells: Fibroblasts, chondrocytes, osteocytes.

Common Joint Injuries

• Tendinitis: Inflammation of tendons.

• Bursitis: Inflammation of bursae (fluid-filled sacs).

Cause of Gout

Gout is caused by the accumulation of uric acid crystals in joints, leading to inflammation and pain.

Technical Term for Dislocation

The technical term for joint dislocation is luxation.

Summary Table: Joint Types and Features

Key Equations

• Calcium Homeostasis:

$\text{Blood Ca}^{2+} \uparrow \Rightarrow \text{Calcitonin secretion} \Rightarrow \text{Osteoclast inhibition}$ $\text{Blood Ca}^{2+} \downarrow \Rightarrow \text{PTH secretion} \Rightarrow \text{Osteoclast stimulation}$

Additional info: Some details, such as the technical term for dislocation (luxation), and the summary table, were inferred for completeness and clarity.