Chapter 6: Bone and Skeletal Tissues

Introduction

This chapter explores the structure, function, and classification of bone and skeletal tissues, including cartilage types, bone markings, histology, and common bone diseases. Understanding these foundational concepts is essential for students of Anatomy & 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 and explain the types of cells found in bone.

• 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 type of connective tissue found throughout the body, providing support and flexibility to various structures. It is avascular and consists of chondrocytes embedded in an extracellular matrix.

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

• Functions: Provides support, flexibility, and smooth surfaces for joint movement.

Types of Cartilages

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 cartilage of joints, costal cartilage, nose, trachea, and larynx.

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

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

Comparison Table: Types of Cartilage

Functions of Bones

Major Functions

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

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

• Movement: Acts as levers for muscles to produce movement.

• Mineral Storage: Stores calcium and phosphate ions, releasing them as needed.

• Blood Cell Formation: Houses red bone marrow, the site of hematopoiesis (production of blood cells).

• Fat Storage: Yellow bone marrow stores triglycerides.

• Hormone Production: Produces osteocalcin, which helps regulate bone formation and metabolism.

Bone Classification

Types of Bones

Bones are classified by shape and structure:

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

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

• Flat Bones: Thin, flattened, and often curved (e.g., sternum, scapula, ribs).

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

Bone Markings

Types and Functions

Bone markings are surface features that serve as sites for muscle, ligament, and tendon attachment, or as conduits for nerves and blood vessels.

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

Compact vs. Spongy Bone

Structural Differences

• Compact Bone: Dense, solid outer layer; provides strength and protection. Contains osteons (Haversian systems).

• Spongy Bone (Cancellous Bone): Porous, inner layer; contains trabeculae and spaces filled with bone marrow.

Bone Composition

Organic and Inorganic Components

• Organic: Mostly collagen fibers produced by osteoblasts; provides flexibility and tensile strength.

• Inorganic: Mainly hydroxyapatite (calcium phosphate crystals); provides hardness and resistance to compression.

Formula for Hydroxyapatite:

Bone Cell Types

Major Cell Types

• Osteogenic Cells: Stem cells that differentiate into osteoblasts.

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

• Osteocytes: Mature bone cells; maintain bone tissue.

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

Histology of Bone

Compact Bone Structure

• Osteon (Haversian System): Structural unit of compact bone; 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 those in central canals.

• Lacunae: Small spaces housing osteocytes.

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

Spongy Bone Structure

• Trabeculae: Lattice-like network of bone tissue; spaces filled with red or yellow marrow.

Bone Ossification

Types of Ossification

• Intramembranous Ossification: Bone develops from fibrous membranes; forms flat bones (e.g., skull, clavicle).

• Endochondral Ossification: Bone forms by replacing hyaline cartilage; forms most bones of the skeleton.

Bone Growth

Growth Mechanisms

• Interstitial Growth: Lengthening of bones via cartilage growth at the epiphyseal plate.

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

Bone Disorders

Rickets

• Definition: Childhood disease caused by vitamin D deficiency, leading to soft, poorly mineralized bones.

• Symptoms: Bone pain, deformities, delayed growth.

• Treatment: Vitamin D and calcium supplementation.

Osteoporosis

• Definition: Condition characterized by decreased bone mass and increased fragility.

• Risk Factors: Age, gender (more common in postmenopausal women), genetics, low calcium/vitamin D intake, sedentary lifestyle.

• Treatment: Medications (bisphosphonates), calcium and vitamin D supplements, weight-bearing exercise.

Paget's Disease

• Definition: Chronic disorder causing abnormal bone remodeling, resulting in enlarged and misshapen bones.

• Symptoms: Bone pain, deformities, increased fracture risk.

Example: Osteoporosis is commonly seen in elderly women and can lead to hip fractures after minor falls.