Cartilage and Bone Tissues: Structure, Function, and Development

Cartilage & Bone Tissues

Structure of Cartilage

Cartilage is a specialized connective tissue that serves as a flexible, resilient structure between dense connective tissue and bone. It is avascular and lacks nerve fibers, making it slow to heal.

• Avascular: No blood vessels or nerves present.

• Ground Substance: Contains large amounts of glycosaminoglycans (GAGs) such as hyaluronic acid, which help retain water.

• Collagen Fibers: Varying amounts of elastic fibers provide flexibility and strength.

• Cartilage can be up to 80% water, contributing to its resilience.

Key Terms Related to Cartilage

• Perichondrium: Dense connective tissue layer surrounding cartilage; supplies nutrients and aids in repair. Poor vascularization leads to slow healing and scar tissue formation.

• Chondroblasts: Immature cartilage cells that actively form cartilage matrix.

• Chondrocytes: Mature cartilage cells that maintain the cartilage matrix.

• Lacunae: Small cavities within cartilage that house chondrocytes.

Types of Cartilage

• Hyaline Cartilage: Most common type; provides firm support and pliability.

  • Contains abundant collagen fibers; appears glassy (blue-white).

  • Chondrocytes occupy 1-10% of tissue volume.

  • Location: Embryonic skeleton, ends of long bones (epiphyseal plates), costal cartilage, nose, trachea, larynx.

  • Function: Supports and reinforces; resists compressive stress.

• Elastic Cartilage: Similar to hyaline cartilage but with more elastic fibers.

  • Location: External ear, epiglottis.

  • Function: Maintains shape while allowing flexibility.

• Fibrocartilage: Rows of chondrocytes alternating with thick collagen fibers; intermediate between hyaline cartilage and dense regular connective tissue.

  • Location: Intervertebral discs, pubic symphysis, menisci of knee joints.

  • Function: Provides tensile strength and absorbs compressive shock.

Functions of Bone Tissue

Bones are dynamic organs that serve multiple essential functions in the body:

1. Support: Provides structural framework for the body.

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

3. Anchorage & Movement: Serves as levers for muscles to produce movement.

4. Mineral Storage: Stores minerals such as calcium and phosphorus.

5. Blood Cell Formation: Hematopoiesis occurs in red bone marrow.

6. Fat Storage: Yellow bone marrow stores triglycerides after growth ceases.

7. Hormone Production: Osteocalcin influences metabolism and insulin regulation.

Classification of Bones

Bones are classified by shape and structure:

• Long Bones: Longer than they are wide; consist of a shaft and two ends (e.g., femur, humerus).

• Short Bones: Cube-shaped; found in the wrist and ankle (e.g., carpals, tarsals).

• Flat Bones: Thin, flattened, and usually curved (e.g., sternum, ribs, skull bones).

• Irregular Bones: Complicated shapes (e.g., vertebrae, hip bones).

Microscopic Structure of Bone Tissue

Compact Bone

• Osteon (Haversian System): Structural unit; elongated cylinder parallel to the long axis of bone.

• Lamellae: Concentric rings of bone matrix.

• Central Canal: Contains blood vessels and nerves.

• Lacunae: Small spaces housing osteocytes.

• Canaliculi: Tiny canals connecting lacunae, allowing for nutrient and waste exchange.

• Interstitial Lamellae: Fill gaps between osteons.

• Circumferential Lamellae: Extend around the entire circumference of the diaphysis.

Spongy Bone

• Composed of trabeculae (lattice-like network).

• Osteocytes in trabeculae are interconnected by canaliculi.

• No osteons present.

• Nutrients diffuse through canaliculi from marrow spaces.

Structure of Long Bones

• Diaphysis: Tubular shaft; collar of compact bone surrounding the medullary cavity (marrow cavity).

• Epiphyses: Rounded ends; exterior is compact bone, interior is spongy bone; covered by articular (hyaline) cartilage.

• Epiphyseal Line: Remnant of the epiphyseal plate (growth plate) between diaphysis and epiphysis.

Microscopic Structure of Other Bone Types

• All bone types have a compact bone exterior and spongy bone interior.

• Covered with periosteum (outer layer) and endosteum (lines internal surfaces).

Bone Cells

• Osteocytes: Mature bone cells in lacunae; maintain bone matrix.

• Osteoblasts: Bone-forming cells; secrete bone matrix.

• Osteoclasts: Bone-resorbing cells; break down bone matrix.

Bone Formation and Remodeling

Bone develops through two main processes:

1. Intramembranous Ossification:

   • Bone develops from fibrous connective tissue membranes.

   • Forms flat bones of the skull and clavicles.

   • Begins around the 8th week of embryonic development.

2. Endochondral Ossification:

   • Bone develops by replacing hyaline cartilage.

   • Forms most bones below the skull (except clavicles).

   • Begins in the second month of development; more complex process.

Summary Table: Types of Cartilage

Type	Main Features	Location	Function
Hyaline	Most common; glassy matrix, few fibers	Ends of long bones, nose, trachea, larynx, costal cartilage	Support, flexibility, reduces friction
Elastic	More elastic fibers than hyaline	External ear, epiglottis	Maintains shape, flexibility
Fibrocartilage	Thick collagen fibers, rows of chondrocytes	Intervertebral discs, pubic symphysis, menisci	Tensile strength, absorbs shock

Summary Table: Bone Cells

Cell Type	Function
Osteoblast	Bone formation; secretes bone matrix
Osteocyte	Maintains bone tissue
Osteoclast	Resorbs (breaks down) bone

Key Equations

• Bone Strength (proportional to density and cross-sectional area): $\text{Strength} \propto \text{Density} \times \text{Area}$

Additional info: Some explanations and definitions have been expanded for clarity and completeness based on standard Anatomy & Physiology curriculum.