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

Bone and Skeletal Tissues

This chapter introduces the structure and function of bone and skeletal tissues, focusing on cartilage types, bone classification, histology, and common bone diseases. Understanding these foundational concepts is essential for the study of the musculoskeletal system in 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 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 specialized connective tissue that provides support and flexibility to various parts of the body. 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 of knee joints.

• Functions: Provides flexible support, reduces friction at joints, and absorbs shock.

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. Contains fine collagen fibers and chondrocytes in lacunae. Found in articular cartilage of joints, costal cartilage, nose, trachea, and larynx.

• Elastic Cartilage: Contains abundant elastic fibers in addition to collagen. Provides strength and elasticity. 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 of the knee.

Type	Main Fibers	Location	Function
Hyaline	Fine collagen	Joints, ribs, nose, trachea	Support, flexibility, reduces friction
Elastic	Elastic fibers	Ear, epiglottis	Maintains shape, flexibility
Fibrocartilage	Thick collagen	Intervertebral discs, pubic symphysis, menisci	Shock absorption, tensile strength

Functions of Bones

Major Functions

Bones serve multiple vital functions in the human body:

• Support: Provide a framework for the body and support soft tissues.

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

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

• Mineral Storage: Store minerals, primarily calcium and phosphate, which can be released into the bloodstream as needed.

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

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

• Hormone Production: Osteocalcin, produced by bones, helps regulate bone formation and energy metabolism.

Bone Classification

Types of Bones

Bones are classified by shape and structure:

• Long Bones: Longer than they are wide; consist of a shaft and two ends (e.g., femur, humerus).

• Short Bones: Cube-shaped; found in the wrist (carpals) and ankle (tarsals).

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

• Irregular Bones: Complicated 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, 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, sinus).

Compact vs. Spongy Bone

Structural Differences

• Compact Bone: Dense, solid outer layer; provides strength and protection. Contains osteons (Haversian systems) with concentric lamellae around central canals.

• Spongy Bone (Cancellous Bone): Internal network of trabeculae; spaces filled with red or yellow marrow. Provides lightweight support and houses 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.

Formula for Hydroxyapatite:

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

Bone Cell Types

Major Cell Types

• Osteogenic Cells: Stem cells that differentiate into osteoblasts.

• Osteoblasts: Bone-forming cells; secrete osteoid.

• Osteocytes: Mature bone cells; maintain bone matrix and communicate via canaliculi.

• Osteoclasts: Bone-resorbing cells; break down bone matrix for remodeling and calcium release.

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/waste exchange.

Spongy Bone Structure

• Trabeculae: Lattice-like network; no osteons.

• Spaces: Filled with red or yellow marrow.

Bone Ossification

Types of 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 bones of the skeleton, especially long bones.

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 new bone tissue at 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), low calcium intake, sedentary lifestyle.

• Treatment: Calcium and vitamin D supplementation, weight-bearing exercise, medications (e.g., bisphosphonates).

Summary Table: Cartilage Types

Type	Main Fiber	Location	Function
Hyaline	Collagen	Joints, ribs, nose, trachea	Support, flexibility
Elastic	Elastic fibers	Ear, epiglottis	Shape, flexibility
Fibrocartilage	Thick collagen	Intervertebral discs, menisci	Shock absorption

Additional info: This guide expands on brief notes and slide content to provide a comprehensive, exam-ready summary of bone and skeletal tissue structure and function.