Cartilage and Bone Tissues

Overview

This section explores the structure, types, and functions of cartilage and bone tissues, which are essential components of the human skeletal system. Understanding their differences, microscopic anatomy, and physiological roles is fundamental for students of Anatomy & Physiology.

Differences Between Bone and Cartilage

General Comparison

• Bone is a rigid, highly vascularized tissue that provides structural support, protection, and serves as a reservoir for minerals.

• Cartilage is a flexible, avascular tissue that cushions joints and maintains the shape of certain structures.

• Bone contains nerves and blood vessels; cartilage does not.

• Bone matrix is mineralized (mainly calcium phosphate); cartilage matrix is rich in glycosaminoglycans and water.

Structure and Types of Cartilage

General Structure

• Cartilage is tough but flexible, intermediate in structure between dense connective tissue and bone.

• It is avascular (lacks blood vessels) and devoid of nerve fibers.

• The ground substance contains glycosaminoglycans (GAGs) such as chondroitin sulfate and hyaluronic acid, and proteins like chondronectin.

• Contains collagen fibers and sometimes elastic fibers.

• Composed of up to 80% water, contributing to its resilience.

Key Terms Related to Cartilage

• Perichondrium: Dense layer of connective tissue surrounding cartilage; aids in growth and repair.

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

• Chondrocytes: Mature cartilage cells that maintain the cartilage matrix; found in clusters called lacunae.

Types of Cartilage

• Hyaline Cartilage: Most abundant type; provides firm support with pliability. Appears glassy blue-white. Locations: embryonic skeleton, epiphyseal plates, costal cartilages of ribs, nose, trachea, larynx. Functions: supports, cushions, resists compressive stress.

• Elastic Cartilage: Similar to hyaline but contains more elastic fibers. Locations: external ear, epiglottis. Functions: maintains shape and provides flexibility.

• Fibrocartilage: Rows of chondrocytes alternating with thick collagen fibers; intermediate between hyaline cartilage and dense regular connective tissue. Locations: intervertebral discs, pubic symphysis, knee joint discs. Functions: tensile strength and ability to absorb compressive shock.

Functions and Classification of Bone Tissue

Functions of Bone

• Support: Provides structural framework for the body.

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

• Anchorage & Movement: Serves as attachment points for muscles, enabling movement.

• Mineral Storage: Stores minerals, especially calcium and phosphorus.

• Blood Cell Formation: Houses bone marrow, which produces blood cells (hematopoiesis).

• Fat Storage: Yellow marrow stores fat.

• Hormone Production: Produces osteocalcin, which regulates bone formation and influences energy metabolism.

Classification of Bones

• Long Bones: Longer than wide; shaft (diaphysis) and two ends (epiphyses). Examples: femur, humerus.

• Short Bones: Cube-shaped; mostly spongy bone. Examples: wrist (carpals), ankle (tarsals).

• Flat Bones: Thin, flattened, and often curved. Examples: skull bones, ribs, sternum.

• Irregular Bones: Complex shapes. Examples: vertebrae, hip bones.

Microscopic Structure of Bone Tissue

Compact (Cortical) Bone

• Osteon (Haversian System): Structural unit; elongated cylinders parallel to bone axis.

• Lamellae: Concentric rings of bone matrix.

• Central (Haversian) Canal: Contains blood vessels and nerves.

• Perforating (Volkmann's) Canals: Perpendicular to long axis; connect blood and nerve supply.

• Lacunae: Small cavities housing osteocytes.

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

• Interstitial Lamellae: Fill gaps between osteons.

• Circumferential Lamellae: Encircle the entire bone shaft.

Spongy (Trabecular) Bone

• Composed of trabeculae (small, needle-like pieces of bone).

• Contains lamellarly arranged osteocytes and canaliculi.

• No osteons; nutrients diffuse through canaliculi from marrow spaces.

• Trabeculae are arranged along lines of stress for strength.

Bone Cells

• Osteoprogenitor Cells: Stem cells that differentiate into osteoblasts.

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

• Osteocytes: Mature bone cells; maintain bone tissue.

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

Structure of Long Bones

• Diaphysis: Shaft; composed of compact bone surrounding the medullary cavity (contains yellow marrow).

• Epiphyses: Ends of long bone; spongy bone interior, thin layer of compact bone outside, covered by articular (hyaline) cartilage.

• Epiphyseal Line: Remnant of the epiphyseal plate (growth plate).

Bone Formation and Growth

Ossification (Osteogenesis)

• Process of bone formation, including embryonic development, growth, and remodeling.

• Intramembranous Ossification: Bone develops from fibrous connective tissue membranes; forms flat bones (skull, clavicles).

• Endochondral Ossification: Bone develops by replacing hyaline cartilage; forms most bones below the skull.

Mechanisms of Bone Growth

• Interstitial Growth: Lengthening of long bones at the epiphyseal plate.

• Appositional Growth: Increase in bone thickness by addition of bone matrix to the surface.

• Growth in length ceases when the epiphyseal plate closes (about age 18 in females, 21 in males).

Bone Remodeling and Repair

Remodeling

• Continuous process of bone resorption (by osteoclasts) and deposition (by osteoblasts).

• Maintains bone strength and mineral homeostasis.

Fracture Repair

1. Hematoma Formation: Blood clot forms; bone cells deprived of oxygen die.

2. Fibrocartilaginous Callus Formation: Blood vessels invade; macrophages clean up debris; fibroblasts, chondroblasts, and osteoblasts lay down new matrix.

3. Bony Callus Formation: Cartilage is replaced by trabecular bone.

4. Remodeling: Excess bone is removed; compact bone is restored.

Hormonal Regulation of Bone Density

Parathyroid Hormone (PTH)

• PTH is released when blood calcium levels are low.

• Stimulates osteoclasts to resorb bone, releasing calcium into the bloodstream.

• Maintains calcium homeostasis.

Osteoporosis

Overview

• Occurs when bone resorption outpaces bone formation, leading to porous and fragile bones.

• Commonly affects the spine and neck of the femur.

• Risk factors: insufficient exercise, poor diet (low calcium/protein), abnormal vitamin D receptors, smoking, hormonal changes (reduced estrogen/testosterone).

Bone Markings

Types and Functions

• Foramen: Round or oval opening through a bone.

• Fossa: Indentation at the edge of a structure.

• Process: Very large, blunt, irregularly shaped projection.

Summary Table: Types of Cartilage

Key Equations

• Bone Density Regulation:

Additional info: Academic context and definitions have been expanded for clarity and completeness.