Structure and Coverings of Bone and Cartilage

Layers and Cell Types

The coverings of bone and cartilage provide protection, nourishment, and facilitate growth and repair. Each has distinct layers and cell types.

• Periosteum: Dense connective tissue covering bones; contains osteoblasts (bone-forming cells).

• Endosteum: Thin membrane lining the medullary cavity; contains osteoprogenitor cells.

• Perichondrium: Surrounds cartilage; contains chondroblasts (cartilage-forming cells).

• Cell Types: Osteoblasts (form bone), osteocytes (maintain bone), osteoclasts (resorb bone), chondroblasts (form cartilage), chondrocytes (maintain cartilage).

Cartilage Growth

Interstitial vs. Appositional Growth

Cartilage grows by two mechanisms, each contributing to its development and repair.

• Interstitial Growth: Chondrocytes divide and secrete matrix from within; increases length.

• Appositional Growth: New layers added by chondroblasts in the perichondrium; increases thickness.

Bone Marrow

Bone marrow is a soft tissue found in the cavities of bones, responsible for hematopoiesis (blood cell formation).

• Red Marrow: Produces red and white blood cells.

• Yellow Marrow: Stores fat; found in adult long bones.

Types of Bone

Spongy vs. Compact Bone

Bones are classified based on their structure and density.

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

• Spongy Bone: Porous, found at ends of long bones; contains trabeculae and red marrow.

Vascularization of Bone

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

Bone Cells

Blast vs. Cyte vs. Clast

Bone cells have specialized functions in bone formation, maintenance, and resorption.

• Osteoblasts: Synthesize bone matrix.

• Osteocytes: Mature bone cells, maintain bone tissue.

• Osteoclasts: Break down bone matrix for remodeling.

Ossification

Endochondral vs. Intramembranous Ossification

Bone formation occurs via two main processes:

• Endochondral Ossification: Bone develops from hyaline cartilage model; 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 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

• Brings blood vessels, osteoblasts, and osteoclasts into the cartilage model, initiating bone formation.

How Spongy Bone Forms in Intramembranous Ossification

• Mesenchymal cells differentiate into osteoblasts, which secrete matrix forming trabeculae (spongy bone).

Bone Growth

Appositional vs. Interstitial Growth

Bones grow in diameter (appositional) and length (interstitial).

• Appositional Growth: Osteoblasts add new bone to the surface, increasing thickness.

• Interstitial Growth: Occurs at the epiphyseal plate, increasing length.

Functional Zones of the Growth (Epiphyseal) Plate

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

• Resting Zone: Quiescent chondrocytes.

• Proliferation Zone: Rapidly dividing chondrocytes.

• Hypertrophic Zone: Enlarged chondrocytes.

• Calcification Zone: Matrix becomes calcified; chondrocytes die.

• Ossification Zone: Osteoblasts lay down new bone.

Hormones Affecting Bone Growth

Several hormones regulate bone growth and remodeling:

• Growth Hormone: Stimulates bone growth at the epiphyseal plate.

• Thyroid Hormone: Modulates growth hormone effects.

• Sex Hormones: Promote growth spurt and epiphyseal plate closure during puberty.

Substances Used by Osteoclasts

Osteoclasts resorb bone using acids and enzymes:

• Hydrochloric Acid (HCl): Dissolves mineral components.

• Lysosomal Enzymes: Digest organic matrix.

Wolff's Law

Wolff's Law states that bone adapts to the mechanical stresses placed upon it, becoming stronger with increased load.

Bone and Calcium Homeostasis

Role of Calcitonin & Parathyroid Hormone, Feedback Mechanism

Calcium levels in blood are tightly regulated by hormones:

• Parathyroid Hormone (PTH): Increases blood calcium by stimulating osteoclasts.

• Calcitonin: Lowers blood calcium by inhibiting osteoclasts.

Feedback Mechanism:

• Low blood calcium → PTH release → bone resorption → increased blood calcium.

• High blood calcium → Calcitonin release → bone deposition → decreased blood calcium.

Fracture Classification and Common Types

Fractures are classified by their pattern and cause:

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

• Compound (Open): Bone breaks and pierces skin.

• Comminuted: Bone shatters into multiple pieces.

• Greenstick: Incomplete break, common in children.

• Spiral: Twisting force causes break.

Fracture Healing (Hematoma, Callus)

Bone healing occurs in stages:

• Hematoma Formation: Blood clot forms at fracture site.

• Fibrocartilaginous Callus: Soft callus forms, bridging the gap.

• Bony Callus: Hard callus replaces soft callus.

• Bone Remodeling: Restores bone to original shape.

Osteoporosis vs. Osteomalacia

Both are bone disorders but differ in cause and presentation.

• Osteoporosis: Decreased bone density; increased fracture risk.

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

Joint Structure and Movement Classifications

Joints are classified by structure and movement:

• Fibrous Joints: Bones joined by dense connective tissue; little movement (e.g., sutures).

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

• Synovial Joints: Bones separated by a fluid-filled cavity; freely movable.

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

• Reduces friction and absorbs shock in synovial joints.

Synovial Joint Structure and Features

Synovial joints are the most movable type of joint, characterized by:

• Articular Cartilage: Covers bone ends.

• Joint (Synovial) Cavity: Contains synovial fluid.

• Articular Capsule: Encloses joint cavity.

• Synovial Fluid: Lubricates and nourishes cartilage.

• Ligaments: Strengthen and stabilize joint.

Shoulder Joint Features

• Ball-and-socket joint; allows greatest range of motion.

• Stabilized by rotator cuff muscles and ligaments.

Sprain vs. Dislocation

• Sprain: Ligament injury without bone displacement.

• Dislocation: Bones forced out of normal position in joint.

Composition of Connective Tissue Matrices

Connective tissues have specialized matrices:

• Bone Matrix: Collagen fibers, hydroxyapatite crystals.

• Cartilage Matrix: Collagen, proteoglycans, water.

Common Joint Injuries

• Tendinitis: Inflammation of tendons.

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

Cause of Gout

Gout is a joint disorder caused by deposition of uric acid crystals in joints, leading to inflammation and pain.

Summary Table: Joint Types and Features

Key Equations

• Calcium Homeostasis:

Additional info: Some details, such as the stages of fracture healing and the composition of connective tissue matrices, were expanded for academic completeness.