Cartilage

Composition and Structure

Cartilage is a specialized connective tissue that provides support and flexibility to various parts of the body. It is primarily composed of:

• Bone salts: Mineral components that contribute to rigidity (mainly in bone, less in cartilage).

• Water: Major component, providing resilience and flexibility.

• Blood vessels: Cartilage is avascular (lacks blood vessels).

• Nerves: Cartilage is aneural (lacks nerves).

Additional info: Cartilage matrix consists mainly of collagen fibers and proteoglycans.

Perichondrium

The perichondrium is a dense layer of connective tissue surrounding cartilage (except at articular surfaces). Its functions include:

• Nourishing cartilage and resisting expansion.

• Forming lacunae: Small spaces housing chondrocytes.

• Producing collagen and replacing osteons (in bone tissue).

Chondrocytes

Chondrocytes are the primary cells in cartilage, responsible for maintaining the cartilage matrix.

• Located in lacunae (small cavities within the matrix).

• Derived from bone stem cells.

• Different from fibroblasts and osteoclasts (bone-resorbing cells).

Types of Cartilage

Cartilage types are classified based on fiber composition:

• Hyaline cartilage: Contains only collagen fibers; most abundant type.

• Fibrocartilage: Contains collagen and elastic fibers; found in intervertebral discs.

• Elastic cartilage: Contains elastic fibers; found in ear and epiglottis.

Growth of Cartilage

Appositional vs. Interstitial Growth

Cartilage grows by two mechanisms:

• Appositional growth: Addition of new matrix by perichondrium cells at the external surface.

• Interstitial growth: Division of chondrocytes within lacunae, expanding cartilage from within.

Functions of Bones

Main Functions

Bones serve several vital functions in the body:

• Support: Framework for the body.

• Protection: Shields vital organs (e.g., skull protects brain).

• Hormone production: Osteocalcin secretion regulates metabolism.

• Digestion: Not a direct function of bones.

Classification of Bones

Number and Types

• Named bones in the human skeleton: 206.

• Axial skeleton: Includes skull, vertebral column, and rib cage; excludes limbs.

• Sesamoid bones: Bones embedded in tendons (e.g., patella).

Bone Cells

Types and Functions

• Osteoblasts: Secrete bone matrix (hydroxyapatite).

• Osteocytes: Mature bone cells located in lacunae; maintain bone tissue.

• Osteoclasts: Resorb bone tissue during remodeling.

• Chondrocytes: Cartilage cells, not involved in bone resorption.

Bone Matrix

• Hydroxyapatite: Main mineral component of bone matrix.

• Cartilage matrix: Produced by chondrocytes.

Bone Development

Ossification

• Ossification: Process of bone tissue formation.

• Intramembranous ossification: Forms flat bones (e.g., skull, clavicle).

• Endochondral ossification: Forms most bones, including long bones.

Bone Growth

Epiphyseal Plate Zones

The epiphyseal plate (growth plate) has several zones:

• Resting zone: Inactive cartilage.

• Proliferation zone: Rapid cell division.

• Hypertrophic zone: Enlarged chondrocytes.

• Calcification zone: Matrix becomes calcified.

Hormonal Regulation

• Growth hormone: Stimulates epiphyseal plate activity in childhood.

• Parathyroid hormone (PTH): Stimulates osteoclasts to raise blood calcium.

• Calcitonin: Lowers blood calcium by inhibiting osteoclasts.

• Estrogen and testosterone: Regulate bone growth during adolescence.

Bone Remodeling and Repair

Remodeling

• Bones are remodeled by osteoblasts and osteoclasts.

• Remodeling responds to stress and nutritional needs.

Repair Stages

• Hematoma formation: Blood clot at fracture site.

• Fibrocartilaginous callus formation: Temporary cartilage bridge.

• Bone deposition: New bone formation.

• Bone ends retain normal position: Called nondisplaced fracture.

• Open fracture: Bone ends penetrate the skin.

Summary Table: Bone Cells and Functions

Key Equations

• Hydroxyapatite formula: