Chapter 6 – Introduction to Bones and Bone Tissue: Study Guide

Chapter 6 – Introduction to Bones and Bone Tissue

Overview

This chapter introduces the structure, function, and physiology of bones and bone tissue. It covers bone anatomy, types of bone cells, bone growth and development, bone remodeling, and fracture healing.

Key Terms and Definitions

• Diaphysis: The shaft or central part of a long bone, primarily composed of compact bone that surrounds the medullary cavity.

• Epiphysis: The rounded end of a long bone, typically filled with spongy bone and red marrow.

• Marrow (Red and Yellow): Red marrow is involved in hematopoiesis (blood cell formation) and is found mainly in flat bones and the epiphyses of long bones. Yellow marrow consists mostly of adipose tissue and is found in the medullary cavity of long bones.

• Osteon: The structural unit of compact bone, also called a Haversian system, consisting of concentric lamellae surrounding a central canal.

• Lamellae: Layers of bone matrix in which collagen fibers and mineral crystals align in parallel, providing strength.

• Osteogenesis/Ossification: The process of bone formation, including both intramembranous and endochondral ossification.

• Calcification: The deposition of calcium salts within a tissue, typically referring to the hardening of bone matrix.

• Fontanelles: Soft spots on a baby's skull where the bones have not yet fused, allowing for growth of the brain and skull.

• Longitudinal Growth: Increase in bone length, primarily occurring at the epiphyseal plates through endochondral ossification.

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

• Bone Deposition: The process by which osteoblasts lay down new bone matrix.

• Bone Resorption: The process by which osteoclasts break down bone tissue, releasing minerals into the blood.

• Hematoma: A localized collection of blood outside blood vessels, often occurring after bone fracture as the first step in healing.

• Bone Callus: A mass of tissue that forms at a fracture site and connects the broken ends of the bone during healing.

Bone Structure and Function

Functions of the Skeletal System

• Support: Provides structural framework for the body.

• Protection: Shields vital organs (e.g., skull protects the brain).

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

• Mineral Storage: Stores calcium and phosphate ions.

• Blood Cell Production: Houses red marrow for hematopoiesis.

• Triglyceride Storage: Stores fat in yellow marrow.

Periosteum

• Function: Covers the outer surface of bones, providing nourishment, protection, and a site for muscle attachment.

• Composition: Outer fibrous layer (dense irregular connective tissue) and inner osteogenic layer (contains osteoblasts and osteoclasts).

Perforating (Sharpey's) Fibers

• Collagen fibers that anchor the periosteum to the underlying bone, providing structural stability.

Compact vs. Spongy Bone

• Compact Bone: Dense, forms the outer layer of bones, composed of osteons.

• Spongy Bone: Porous, found at the ends of long bones and inside flat bones, composed of trabeculae.

• Comparison: Compact bone provides strength for weight-bearing; spongy bone reduces bone weight and contains red marrow.

Epiphyseal Lines and Plates

• Epiphyseal Plate: Hyaline cartilage plate in growing bones, site of longitudinal growth.

• Epiphyseal Line: Remnant of the plate in adult bones, indicating where growth has ceased.

Sinuses in Bones

• Air-filled spaces within certain bones (e.g., skull) that reduce bone weight and contribute to voice resonance.

Blood Supply to Bones

• Short, flat, irregular, long, and sesamoid bones receive blood from nutrient arteries, periosteal vessels, and metaphyseal/epiphyseal arteries.

Red and Yellow Bone Marrow

• Red Marrow: Produces red and white blood cells and platelets; found in spongy bone.

• Yellow Marrow: Stores fat; found in the medullary cavity of long bones.

Extracellular Matrix (ECM) of Bone

• Organic Component: Collagen fibers and ground substance (provides flexibility).

• Inorganic Component: Hydroxyapatite crystals (calcium phosphate) provide hardness.

Bone Cells and Their Functions

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

• Osteocytes: Mature bone cells; maintain bone tissue.

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

Central Canals and Canaliculi

• Central (Haversian) Canals: Contain blood vessels and nerves within osteons.

• Canaliculi: Tiny channels connecting osteocytes, allowing for nutrient and waste exchange.

Trabeculae

• Irregular latticework of thin bony plates in spongy bone, providing structural support and housing marrow.

Bone Development and Growth

Intramembranous vs. Endochondral Ossification

• Intramembranous Ossification: Bone develops directly from mesenchymal tissue (e.g., flat bones of skull).

• Endochondral Ossification: Bone develops by replacing hyaline cartilage (e.g., long bones).

Longitudinal Bone Growth

• Occurs at the epiphyseal plate via proliferation and hypertrophy of chondrocytes, followed by ossification.

• Zones of the epiphyseal plate: resting, proliferative, hypertrophic, calcification, ossification.

Appositional Bone Growth

• Increase in bone diameter by addition of new bone at the periosteal surface.

Hormonal Regulation of Bone Growth

• Growth Hormone (GH): Stimulates bone growth by increasing chondrocyte and osteoblast activity.

• Other Hormones: Thyroid hormone, sex hormones (estrogen, testosterone) also influence bone growth.

Bone Remodeling and Homeostasis

Bone Remodeling

• Continuous process of bone deposition (by osteoblasts) and resorption (by osteoclasts).

• Functions: Adaptation to stress, repair of microdamage, regulation of calcium levels.

Calcium Homeostasis

• Bone acts as a reservoir for calcium ions.

• Remodeling maintains blood calcium levels via hormonal control.

Hormonal Control

• Calcitonin: Lowers blood calcium by inhibiting osteoclast activity.

• Parathyroid Hormone (PTH): Raises blood calcium by stimulating osteoclasts and increasing calcium reabsorption in kidneys.

Bone Fractures and Healing

Types of Fractures

• Simple (Closed) Fracture: Bone breaks but does not penetrate the skin.

• Compound (Open) Fracture: Bone breaks and pierces the skin.

• Spiral Fracture: Bone is twisted apart.

• Greenstick Fracture: Incomplete break, common in children.

• Compression Fracture: Bone is crushed, often in vertebrae.

• Avulsion Fracture: Fragment of bone is pulled off by a tendon or ligament.

• Epiphyseal Plate Fracture: Involves the growth plate, can affect bone growth in children.

• Comminuted Fracture: Bone is broken into several pieces.

Steps of Fracture Healing

1. Hematoma formation

2. Fibrocartilaginous callus formation

3. Bony callus formation

4. Bone remodeling

Summary Table: Types of Bone Cells

Cell Type	Function
Osteoblast	Builds bone matrix (bone formation)
Osteocyte	Maintains bone tissue
Osteoclast	Resorbs bone matrix (bone breakdown)

Summary Table: Types of Bone Growth

Growth Type	Description	Location
Longitudinal	Increase in length	Epiphyseal plate
Appositional	Increase in diameter/thickness	Periosteum

Key Equations

• Calcium Homeostasis (simplified):

$\text{Bone Deposition} - \text{Bone Resorption} = \text{Net Bone Mass Change}$

• Relationship of Bone Mass to Mechanical Stress (Wolff's Law):

$\text{Bone adapts to the loads under which it is placed}$

Additional info:

• Some definitions and explanations have been expanded for clarity and completeness.

• Tables have been inferred and constructed to aid comparison and classification.