Chapter 6: Bone and Skeletal Tissues – Structure, Function, and Histology

Bone and Skeletal Tissues

Introduction

The skeletal system is composed of bones and associated tissues, including various types of cartilage. These structures provide support, protection, movement, and play a role in mineral storage and blood cell formation. Understanding the composition and function of bone and cartilage is fundamental in anatomy and physiology.

Learning Objectives

• Describe the three types of cartilages and compare their composition and function.

• List and explain the functions of bones.

• Classify bones by type and provide examples.

• Identify and describe bone markings.

• Distinguish between compact and spongy bone.

• Describe the substances that make up bone and their roles.

• Identify the cell types found in bone tissue.

• Describe bone structure from a histological perspective.

• Explain the processes of intramembranous and endochondral ossification.

• Differentiate between interstitial and appositional bone growth.

• Define rickets and osteoporosis, including causes, risk factors, and treatments.

Cartilages

Overview of Cartilage

Cartilage is a resilient and smooth elastic tissue that covers and protects the ends of long bones at the joints and is a structural component of the rib cage, ear, nose, bronchial tubes, intervertebral discs, and many other body components.

• Locations: External ear, nose, articular surfaces of joints, costal cartilage, intervertebral discs, pubic symphysis, respiratory tubes, menisci of knee joints.

Types of Cartilage

There are three main types of cartilage, each with distinct structural and functional properties:

• Hyaline Cartilage: Most abundant type; provides support with flexibility and resilience. Found in articular surfaces, costal cartilages, nose, trachea, and larynx.

• Elastic Cartilage: Contains more elastic fibers, allowing it to withstand repeated bending. Found in the external ear and epiglottis.

• Fibrocartilage: Contains thick collagen fibers, making it highly compressible and able to resist tension. Found in intervertebral discs, pubic symphysis, and menisci.

Comparison Table: Types of Cartilage

Type	Main Features	Locations
Hyaline	Fine collagen fibers, glassy appearance, chondrocytes in lacunae	Articular surfaces, costal cartilage, nose, trachea, larynx
Elastic	Elastic fibers, flexible, chondrocytes in lacunae	External ear, epiglottis
Fibrocartilage	Thick collagen fibers, less firm matrix, chondrocytes in rows	Intervertebral discs, pubic symphysis, menisci

Functions of Bones

Major Functions

• Support: Provides a framework for the body and supports soft tissues.

• Protection: Protects vital organs (e.g., skull protects the brain, rib cage protects the heart and lungs).

• Movement: Serves as levers for muscles to act upon, enabling movement.

• Mineral Storage: Reservoir for minerals, especially calcium and phosphate.

• Blood Cell Formation: Hematopoiesis occurs in red marrow cavities of certain bones.

• Triglyceride Storage: Fat is stored in yellow bone marrow.

Bone Classification

Types of Bones

• Long Bones: Longer than they are wide; e.g., femur, humerus.

• Short Bones: Cube-shaped; e.g., carpals, tarsals.

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

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

Bone Markings

Types and Functions

• Projections: Sites of muscle and ligament attachment (e.g., tuberosity, crest, trochanter, line, tubercle, epicondyle, spine, process).

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

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

Compact vs. Spongy Bone

Structural Differences

• Compact Bone: Dense outer layer; smooth and solid; composed of osteons (Haversian systems).

• Spongy Bone: Internal layer; honeycomb of trabeculae filled with red or yellow marrow.

Bone Composition

Organic and Inorganic Components

• Organic: Osteoid (collagen fibers and ground substance) produced by osteoblasts; provides flexibility and tensile strength.

• Inorganic: Hydroxyapatite (mineral salts, primarily calcium phosphate); provides hardness and resistance to compression.

Equation for Hydroxyapatite:

$Ca_{10}(PO_4)_6(OH)_2$

Bone Cell Types

• Osteogenic Cells: Stem cells that differentiate into osteoblasts.

• Osteoblasts: Bone-forming cells; secrete osteoid.

• Osteocytes: Mature bone cells; maintain bone matrix.

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

Histology of Bone

Compact Bone

• Osteon (Haversian System): Structural unit; consists of concentric lamellae around a central canal.

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

• Perforating (Volkmann's) Canals: Connect blood vessels and nerves of periosteum to central canals.

• Lacunae: Small spaces containing osteocytes.

• Canaliculi: Tiny canals connecting lacunae; allow for nutrient and waste exchange.

Spongy Bone

• Composed of trabeculae; no osteons.

• Spaces between trabeculae contain marrow.

Bone Ossification

Intramembranous Ossification

• Bone develops from fibrous membrane.

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

Endochondral Ossification

• Bone forms by replacing hyaline cartilage.

• Forms most of the skeleton (e.g., long bones).

Bone Growth

Interstitial Growth

• Growth in length; occurs at epiphyseal plates.

Appositional Growth

• Growth in thickness; occurs throughout life.

Bone Disorders

Rickets

• Softening and weakening of bones in children, usually due to vitamin D deficiency.

• Leads to improper mineralization and deformities.

Osteoporosis

• Loss of bone mass and density, leading to fragile bones.

• Common in elderly, especially postmenopausal women.

• Treatment includes calcium/vitamin D supplementation, weight-bearing exercise, and medications.

Paget's Disease

• Disorder of bone remodeling; excessive bone formation and breakdown.

• Results in enlarged and misshapen bones.

Additional info: The above notes expand on the provided slides by including definitions, examples, and context for each topic, as well as a summary table for cartilage types and a chemical formula for hydroxyapatite.