Cartilage and Bone Tissues

Introduction

This study guide covers the structure, types, and functions of cartilage and bone tissues, as well as their differences, classification, and roles in the human body. These topics are foundational for understanding the skeletal system in Anatomy & Physiology.

Differences Between Bone and Cartilage

Overview

Bone and cartilage are specialized connective tissues with distinct structures and functions. Both contribute to the support and movement of the body, but differ in composition, strength, and flexibility.

• Bone: Rigid, highly vascularized, contains nerves, provides structural support and protection.

• Cartilage: Flexible, avascular, devoid of nerve fibers, provides cushioning and pliability.

Structure of Cartilage

General Features

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

• Avascular: Lacks blood vessels; nutrients diffuse through the matrix.

• Devoid of nerve fibers: No direct innervation.

• Ground substance: Rich in glycosaminoglycans (GAGs) such as chondroitin sulfate and hyaluronic acid, and chondronectin (an adhesive protein).

• Fibers: Mainly collagen, with some elastic fibers depending on type.

• Water content: Up to 80% H2O, contributing to resilience.

Key Terms Relating to Cartilage

• Perichondrium: Dense connective tissue layer surrounding cartilage; provides nutrients and aids in repair.

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

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

• Isogenous groups: Localized clusters of chondrocytes within cartilage.

Types of Cartilage

Hyaline Cartilage

Hyaline cartilage is the most abundant type, providing firm support with pliability.

• Appearance: Glassy, blue-white; chondrocytes comprise only 1-10% of volume.

• Locations: Embryonic skeleton, ends of long bones (epiphyseal plates), costal cartilages of ribs, nose, trachea, larynx.

• Function: Supports and reinforces; cushions and resists compressive stress.

Elastic Cartilage

Elastic cartilage contains more elastic fibers than hyaline cartilage, allowing for greater flexibility.

• Locations: External ear, epiglottis.

• Function: Maintains shape while providing flexibility.

Fibrocartilage

Fibrocartilage is characterized by rows of chondrocytes alternating with thick collagen fibers, providing tensile strength and shock absorption.

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

• Function: Withstands heavy pressure and absorbs compressive shock.

Functions of Bone Tissue

Overview

Bones are living, dynamic tissues that respond to mechanical stress and play multiple roles in the body.

• 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 red marrow for hematopoiesis.

• Fat Storage: Yellow marrow stores triglycerides.

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

Classification of Bones

Overview

Bones are classified by shape and structure, which relate to their functions.

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

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

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

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

Microscopic Structure of Bone Tissue

Compact Bone (Cortical Bone)

Compact bone forms the dense outer layer of bones and is highly organized into structural units called osteons.

• Osteon (Haversian System): Cylindrical structures aligned parallel to the bone's long axis; each consists of concentric lamellae (layers of bone matrix).

• Central (Haversian) Canal: Runs through the center of each osteon, containing blood vessels and nerves.

• Perforating (Volkmann's) Canals: Perpendicular to the long axis, connecting blood and nerve supply between osteons.

• Lacunae: Small cavities housing osteocytes, located between lamellae.

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

• Interstitial Lamellae: Fill gaps between osteons, remnants of old osteons.

• Circumferential Lamellae: Encircle the entire shaft just deep to the periosteum.

Spongy Bone (Trabecular Bone)

Spongy bone consists of a network of trabeculae, providing structural support and housing marrow.

• Trabeculae: Lamellarly arranged structures aligned along lines of stress.

• Osteocytes: Located within trabeculae, interconnected by canaliculi.

• No osteons: Nutrients diffuse from marrow spaces to osteocytes.

Bone Cells

Types and Functions

• Osteoprogenitor Cells: Stem cells that differentiate into osteoblasts.

• Osteoblasts: Bone-forming cells responsible for synthesizing bone matrix.

• Osteocytes: Mature bone cells that maintain bone tissue.

• Osteoclasts: Multinucleated cells that resorb bone matrix.

Structure of Long Bones

Key Regions

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

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

• Epiphyseal Line: Remnant of the epiphyseal plate, marking the site of bone growth in length.

Bone Formation and Growth

Overview

Bone formation (osteogenesis or ossification) occurs during embryonic development, childhood, and throughout life via remodeling and repair.

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

• Endochondral Ossification: Bone forms 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 new bone tissue at the surface.

Bones stop growing in length during adolescence, but some (e.g., nose, lower jaw) may continue to grow slowly throughout life.

Bone Remodeling and Repair

Overview

Bone remodeling is a continuous process involving bone resorption and deposition, maintaining bone strength and mineral homeostasis.

• Remodeling: Osteoclasts resorb bone; osteoblasts deposit new bone.

• Repair: Involves hematoma formation, fibrocartilaginous callus formation, conversion to bony callus, and remodeling to restore original shape.

Hormonal Regulation of Bone Density

Parathyroid Hormone (PTH)

PTH plays a critical role in regulating blood calcium levels and bone density.

• Function: Stimulates osteoclast activity to increase blood calcium by resorbing bone.

• Clinical Note: Excessive resorption leads to osteoporosis, especially in areas like the spine and femoral neck.

• Sex Hormones: Estrogen and testosterone inhibit osteoclasts and promote bone deposition.

Bone Markings

Overview

Bones have various markings that serve as sites for muscle attachment, articulation, and passage of nerves and blood vessels.

• Examples: Indentations, projections, roughened areas, and openings.

Summary Table: Types of Cartilage

Key Equations

• Bone Remodeling Rate:

• Calcium Homeostasis:

Additional info: Some details on bone cell differentiation and bone markings were inferred for completeness.