Physiology Lecture 3: Bone Matrix & Advanced Cell Functions

PHYSIOLOGY LECTURE 3: BONE MATRIX & ADVANCED CELL FUNCTIONS

Juvenile vs Adult Bones

  • Growth Capability

    • Juvenile Bones: Can grow longer due to presence of epiphyseal plates

    • Adult Bones: Cannot grow longer; epiphyseal plates have closed

Plate Structure

  • Epiphyseal Growth Plates: Cartilaginous layers at the ends of long bones

  • Epiphyseal Lines: Scar tissue that replaces growth plates after closure

Closure Timing

  • Juvenile Bones: Not applicable; they are still growing

  • Females: Closure occurs between 14-16 years

  • Males: Closure typically happens in late teens to early 20s

Learning Objectives Checklist

  • Understand the components of bone extracellular matrix (organic & inorganic)

  • Master concepts of compressive vs tensile strength

  • Learn the detailed functions of osteoblasts and osteoclasts

  • Comprehend the integration of bone remodeling

  • Recognize experimental evidence for properties of bone strength

  • Compare and contrast juvenile and adult bone anatomy

Key Differences Between Juvenile and Adult Bones

Important Anatomical Regions

  • Diaphysis:

    • Juvenile Bones: Main shaft of long bone

    • Adult Bones: Same function as juvenile

  • Epiphysis/Epiphyses:

    • End portions of long bones (pronunciation: eh-PIF-ih-sees)

  • Metaphysis:

    • Transition zone between diaphysis and epiphysis

  • Medullary Cavity:

    • Hollow center of diaphysis; contains bone marrow

Bone Structure Visual Study Aid

  • Utilize a detailed diagram of long bone anatomy as a study tip – print it unlabeled to identify regions!

Trabecular Organization

  • Juvenile Bones: Less organized with a "sloppy looking" structure

  • Adult Bones: Highly organized along lines of stress

Medullary Cavity Composition

  • Juvenile Bones: Filled with red bone marrow

  • Adult Bones: Mostly yellow marrow, consisting of fat tissue

Memory Trick

  • "PLATES = GROWTH RATES"

    • Indicates that the presence of epiphyseal plates signifies ongoing growth potential!

BONE EXTRACELLULAR MATRIX: The Foundation

Overview of Bone Matrix

  • Bone Matrix Composition: Consists of Organic Matrix + Inorganic Matrix

    • Analogy: Think of it like steel-reinforced concrete

1. ORGANIC MATRIX Components

  • Ground Substance:

    • Proteoglycan Aggregates:

    • Hybrid sugar-protein complexes

    • Carbohydrates radiate from a protein core; increase viscosity of interstitial fluid

    • Hyaluronic Acid:

    • A carbohydrate that thickens fluid and makes it slippery

    • Provides the "ground" for everything else

  • Type I Collagen Fibers:

    • Known as the "Cables" of the matrix

    • Extremely strong, providing tensile strength and resisting tension forces (pulling, twisting)

    • Same type as fibrocartilage matrix (however, fibrocartilage remains soft)

2. INORGANIC MATRIX: The Rock-Hard Component

  • Hydroxyapatite:

    • Chemical formula: $Ca{10}(PO4)6(OH)2$

    • Calcium phosphate salt crystals that provide compressive strength (resist crushing)

    • This component is what makes bone hard; it is the only part of the bone that remains after death as all else decays!

Key Concept

  • Lab bones are simply "calcium phosphate molds"; they are shadows of living bone which includes blood supply, cells, and marrow.

Matrix Structure: Hierarchical Organization of Bone

  • Demonstrates how collagen and hydroxyapatite organize at multiple levels

Strength Type Definitions

  • Tensile Strength:

    • Definition: Resistance to pulling/stretching forces

    • Provided by Type I collagen fibers in the organic matrix; resists twisting, bending, and pulling apart

  • Compressive Strength:

    • Definition: Resistance to crushing/squashing forces

    • Provided by Hydroxyapatite crystals in the inorganic matrix; resists weight and compression

BONE STRENGTH: The Perfect Combination

Analogy

  • Steel-Reinforced Concrete Analogy

    • Concrete foundation alone: Strong against crushing but brittle

    • Steel cables alone: Flexible but too soft for structural support

    • Combination: Rigid yet flexible; simultaneously provides high strength and slight flexibility

    • In Bone: Hydroxyapatite functions as concrete, while collagen acts as steel cables

Memory Trick

  • "TENSE COLLAGEN, COMPRESS CRYSTALS"

    • Tensile strength comes from collagen (reminiscent of "TENdon")

    • Compressive strength arises from crystals (indicates how compact and hard they are)

Benefits of the Combination

  • Reduced Brittleness: Organic matrix prevents shattering

  • Micro-flexibility: Bone can slightly "squish" when under pressure

  • Fluid Movement: Interstitial fluid can flow between lacunae

  • Optimal Trade-off: Strong enough for support yet light enough for mobility

EXPERIMENTAL EVIDENCE: Proving the Components

Experiment 1: Remove Hydroxyapatite (Acid Treatment)

  • Method:

    1. Obtain a long bone from a cadaver

    2. Soak it in concentrated acid for approximately one week

    3. Replenish acid every 24 hours

  • Result:

    • Bone becomes SO flexible that it can be tied in a knot!

  • Conclusion: Only collagen remains, resulting in pure tensile strength with no compressive strength

Experiment 2: Remove Organic Matrix (Heat Treatment)

  • Method:

    1. Burn away all organic components from the bone

  • Result:

    • Bone becomes extremely brittle and breaks easily

  • Conclusion: Only hydroxyapatite remains, demonstrating pure compressive strength without flexibility

ADVANCED BONE CELL FUNCTIONS

OSTEOBLAST PROCESS: The Builders
From Stem Cell to Builder

  1. Osteoprogenitor Cells (Stem Cells):

    • Located in the inner periosteum layer and appear as "thick Sharpie lines"

    • Always dividing: one remains a stem cell while another becomes an osteoblast

  2. Osteoblasts (Active Builders):

    • Exhibit a plumper, thicker appearance

    • Located between the inner periosteum and the outer bone surface; they originate from endosteum around trabeculae

The Building Process (Step-by-Step)

  • Step 1: Create the Framework

    • Synthesize and secrete Type I collagen

    • Produce organic matrix called osteoid, which serves as the soft, flexible foundation

  • Step 2: The Magic Crystallization

    • Reduce solubility of calcium and phosphate in interstitial fluid

    • As solubility drops, precipitation occurs: small calcium phosphate crystals begin to form

  • Step 3: Positive Feedback Crystal Growth

    • Small crystals attract more calcium phosphate, resulting in the formation of larger crystals which in-turn attract even more

    • The process continues until maximum crystallization is reached, leading to the hard, mineralized bone matrix!

OSTEOCLAST PROCESS: The Controlled Destroyers

  • Giant Multinucleated Cells:

    • Originates from the fusion of white blood cells (mostly monocytes)

    • Size: Much larger than other bone cells

    • Located in periosteum and endosteum (similar to osteoblasts)

    • Main function: Reabsorb or “resorb” bone tissue

The Resorption Mechanism

  • Step 1: Seal the Area

    • Podocytes (cellular “feet”) form around the perimeter of the cell

    • Create a sealed compartment known as Howship's lacuna; this prevents acid from leaking out

  • Step 2: Acidify the Environment

    • Pump hydronium ions ($H_3O^+$) into the sealed area

    • A lower pH dissolves the hydroxyapatite crystals (Keep in mind: $pH = -log_{10}[H^+]$; lower pH indicates more acidity)

  • Step 3: Increase Surface Area

    • Ruffled border on the cell’s bottom increases the surface area

    • More surface area leads to increased acid secretion and enzyme release

  • Step 4: Transport and Disposal

    • Endocytosis: Uptake of dissolved bone components

    • Transcytosis: Transport from one side of the cell to the other

    • Exocytosis: Release into interstitial fluid and subsequently into capillaries

BONE REMODELING: Builders + Destroyers = Better Bones

Kitchen Remodeling Analogy

  • Concept: Necessity of both builders and destroyers

    • When remodeling a kitchen:

    • Destruction Phase: Tearing out old cabinets, counters, appliances

    • Building Phase: Installing new, upgraded components

Memory Trick

  • "YIN-YANG BONE GANG"

    • Osteoblasts and osteoclasts function similarly to yin and yang, opposites that create a perfect balance!

Clinical Connection

  • Calcium Storage Organ: The skeletal system serves as a storage reservoir, where osteoclasts can release calcium phosphate into circulation when needed by other organ systems, while osteoblasts can incorporate excess calcium from meals into new bone matrices.

Remodeling Process Outcomes

  • Result: Leads to better and often larger bones

    • Cannot build new structures on top of old structures (analogy)

In Bone Remodeling

  • Osteoclasts: Remove old, worn-out bone matrix

  • Osteoblasts: Build new, stronger bone matrix

Net Result

  • Overall improvement in bone size, strength, and adaptation to mechanical stress

Multiple Purposes of Bone Remodeling

  1. Growth: Increase bone size during development

  2. Repair: Fix microdamage resulting from daily usage

  3. Calcium Homeostasis: Release calcium between meals and store it after meals

  4. Mechanical Adaptation: Strengthen bones in response to exercise

KEY STUDY STRATEGIES

For Exams, Master These Concepts:

  1. Components of matrix: Understand tensile versus compressive strength

  2. Cell Functions: Distinguish between builder versus destroyer roles and mechanisms

  3. Experimental Evidence: Interpret acid treatment results and their implications

  4. Remodeling Purpose: Understand why both building and destruction are necessary

  5. Master Anatomical Terms: Metaphysis, differences between epiphyseal plates and lines

  6. Age Changes: Awareness of red to yellow marrow conversion

Final Study Tip

  • Print out the bone structure diagram unlabeled and practice identifying all components while explaining their functions. This strategy integrates visual and verbal learning for optimal retention!