Cartilage and Bone Tissues: Structure, Types, and Functions

Cartilage and Bone Tissues

Introduction

This study guide covers the structure, types, and functions of cartilage and bone tissues, as well as their differences, cellular components, and roles in the human body. Understanding these tissues is fundamental for Anatomy & Physiology students, as they form the framework and support systems of the body.

Differences Between Bone and Cartilage

Overview

Bone and cartilage are specialized connective tissues with distinct structures and functions. Both contribute to the skeletal system but differ in composition, strength, flexibility, and repair mechanisms.

• Bone: Rigid, highly vascularized, contains nerve fibers, supports and protects organs, stores minerals, and forms blood cells.

• Cartilage: Tough but flexible, avascular, devoid of nerve fibers, provides cushioning and flexibility at joints and other structures.

Structure of Cartilage

General Features

• Cartilage is a connective tissue with a structure intermediate between dense connective tissue and bone.

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

• The ground substance contains abundant glycosaminoglycans (GAGs) such as chondroitin sulfate and hyaluronic acid, and chondronectin (an adhesive protein).

• Contains collagen fibers (and sometimes elastic fibers).

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

Key Terms Relating to Cartilage

• Perichondrium: Dense layer surrounding cartilage, provides nutrients and aids in repair. In damaged areas, it can form scar tissue, but cartilage repairs poorly.

• Ossification: The process by which cartilage is replaced by bone during growth and development.

• Chondroblasts: Immature cartilage cells that actively form cartilage.

• Chondrocytes: Mature cartilage cells that maintain cartilage, often found in clusters called chondronests.

Types of Cartilage

Classification and Locations

There are three main types of cartilage, each with unique structural features and body locations.

• Hyaline Cartilage: Most abundant type; provides firm support and pliability. Appears glassy blue-white, with chondrocytes making up only 1-10% of volume. Found in embryonic skeleton, ends of long bones (epiphyseal plates), costal cartilages of ribs, nose, trachea, and larynx. Functions in support, reinforcement, cushioning, and resistance to compressive stress.

• Elastic Cartilage: Similar to hyaline but contains more elastic fibers. Located in the external ear and epiglottis, providing shape and flexibility.

• Fibrocartilage: Contains rows of chondrocytes alternating with thick collagen fibers; intermediate between hyaline cartilage and dense regular connective tissue. Found in intervertebral discs, pubic symphysis, and knee joint discs. Provides tensile strength and absorbs compressive shock.

Functions of Bone Tissue

Overview

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

• Support: Provides structural framework for the body.

• Protection: Shields vital organs (e.g., skull protects brain, ribs protect heart and lungs).

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

• Mineral Storage: Stores minerals such as calcium and phosphorus, which can be released into the bloodstream as needed.

• Blood Cell Formation: Houses red marrow, where hematopoiesis (blood cell production) occurs.

• Fat Storage: Yellow marrow stores fat as an energy reserve.

• Hormone Production: Produces osteocalcin, a hormone involved in bone metabolism and regulation of blood sugar.

Classification of Bones

Types Based on Shape

Bones are classified according to their shapes, which relate to their functions.

• Long Bones: Longer than wide, with a shaft and two ends (e.g., femur, humerus). Mostly compact bone with marrow cavity; spongy bone near joints.

• Short Bones: Cube-shaped (e.g., wrist and ankle bones). Primarily spongy bone with a thin outer layer of compact bone.

• Flat Bones: Thin, flattened, and often curved (e.g., skull bones, ribs, sternum). Spongy bone sandwiched between compact bone layers.

• Irregular Bones: Complex shapes (e.g., vertebrae, hip bones). Mainly spongy bone with a covering of compact bone.

Microscopic Structure of Bone Tissue

Compact (Cortical) Bone

Compact bone forms the dense outer layer of bones and is highly organized.

• Osteon (Haversian System): The structural unit, consisting of concentric lamellae (rings) of bone matrix around a central canal.

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

• Perforating (Volkmann's) Canals: Run perpendicular to the long axis, connecting blood and nerve supply of the periosteum to the central canals.

• Lacunae: Small cavities housing osteocytes (mature bone cells).

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

• Interstitial Lamellae: Fill gaps between osteons.

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

Spongy (Trabecular) Bone

Spongy bone is found inside bones, especially at the ends of long bones and in flat bones.

• Composed of trabeculae (small, needle-like pieces of bone) arranged along lines of stress.

• Contains lamellarly arranged osteocytes and canaliculi.

• No osteons; nutrients diffuse through canaliculi 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: Bone-resorbing cells involved in bone remodeling.

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 layer of compact bone, covered by articular (hyaline) cartilage for joint articulation.

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

Bone Formation and Growth

Ossification Processes

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

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

Ossification begins in the embryo and continues through childhood and adolescence. Secondary ossification centers form at the ends of long bones, leaving articular cartilage and epiphyseal plates for continued growth.

Mechanisms of Bone Growth

• Interstitial Growth: Lengthening of long bones at the epiphyseal plate during infancy and youth.

• Appositional Growth: Increase in bone thickness by addition of new bone tissue at the surface.

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

Bone Remodeling and Repair

Remodeling Process

• Bone is continuously remodeled by osteoclasts (resorption) and osteoblasts (deposition).

• Remodeling allows bones to adapt to stress and repair microdamage.

Bone Repair Steps

1. Formation of Hematoma: Blood clot forms at the fracture site; bone cells deprived of oxygen die.

2. Formation of Fibrocartilaginous Callus: Blood vessels invade, macrophages clean up debris, and fibroblasts/chondroblasts lay down collagen and cartilage.

3. Conversion to Bony Callus: Cartilage is replaced by trabecular bone.

4. Remodeling: Excess bone is removed, and compact bone is restored to its original shape.

Hormonal Regulation of Bone Density

Parathyroid Hormone (PTH)

• PTH is released by the parathyroid glands in response to low blood calcium levels.

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

• Maintains calcium homeostasis, essential for nerve and muscle function.

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 include insufficient exercise, poor diet (low calcium/protein), abnormal vitamin D receptors, smoking, and hormonal changes (reduced estrogen/testosterone).

Bone Markings

Types and Functions

• Projections: Sites of muscle and ligament attachment (e.g., tuberosity, crest, trochanter).

• Surfaces: Form joints (e.g., head, facet, condyle).

• Depressions and Openings: Allow passage of nerves and blood vessels (e.g., foramen, groove, notch).

Comparison Table: Bone vs. Cartilage

Feature	Bone	Cartilage
Vascularization	Highly vascularized	Avascular
Cell Types	Osteoblasts, osteocytes, osteoclasts	Chondroblasts, chondrocytes
Matrix Composition	Collagen fibers, calcium salts	Collagen/elastic fibers, GAGs
Repair Ability	Good (remodels well)	Poor (forms scar tissue)
Function	Support, protection, movement, storage	Cushioning, flexibility, support

Key Equations

• Bone Remodeling Rate:

$\text{Rate of bone remodeling} = \text{Bone deposition} - \text{Bone resorption}$

• Calcium Homeostasis:

$\text{Blood Ca}^{2+} \uparrow \Rightarrow \text{PTH secretion} \downarrow$

$\text{Blood Ca}^{2+} \downarrow \Rightarrow \text{PTH secretion} \uparrow$

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