ToB 1.2 Light Microscopy

Scale and Size in Microscopy

• Relationship between units

  • $1\ \text{mm} = 10^3\ \mu\text{m}$

  • $1\ \mu\text{m} = 10^3\ \text{nm}$

  • $1\ \text{mm} = 10^6\ \text{nm}$

• Typical human cell sizes

  • Red blood cell (RBC): $7.2\ \mu\text{m}$

  • Most human cells: $10-20\ \mu\text{m}$

  • Neutrophils: $10-12\ \mu\text{m}$

  • Keratinocytes: $20-30\ \mu\text{m}$

  • Oocytes: ~$100\ \mu\text{m}$

• Relative sizes

  • Used to gauge what Light Microscopy (LM) versus Electron Microscopy (EM) can resolve.

• Importance of images and scale

  • Essential for interpreting histological slides and planning biopsy sampling.

Relationship between mm, µm, and nm

• Practical for converting between scales when interpreting microscope field sizes and cell dimensions.

• Image scale is critical for understanding resolution limits and visible features at given magnification.

Limit of Resolution

• Definition: The minimum distance at which two objects can be distinguished as separate entities.

• Depicted as unresolved (indistinguishable) vs. resolved (seen as distinct) points.

• Practical considerations limit resolving power in LM/EM.

Practical and Theoretical Limits of Resolution

• Wavelength dependence

  • Resolution is inversely proportional to wavelength.

  • Visible light wavelengths: ~$\lambda_{\text{visible}} = 380\ \text{nm} - 700\ \text{nm}$.

  • Electron wavelength at ~100 kV: $\lambda_{e} \approx 0.004\ \text{nm}$.

• Practical limits

  • Light microscope (LM): ~$d_{\text{LM}} \approx 200\ \text{nm}.

  • Electron microscope (EM): ~$d_{\text{EM}} \approx 0.1\ \text{nm}.

• Theoretical limit is often not achieved due to aberrations, sample prep, and imaging conditions.

LM vs EM: Basic Architecture and Capabilities

• Light Microscopy (LM)

  • Uses visible light and optical lenses.

  • Key components: lamp, condenser lens, objective lens, eyepiece/ocular, specimen.

  • Excellent for routine histology, histochemical staining (H&E, PAS), and larger structures.

• Electron Microscopy (EM)

  • Uses electron beams and electromagnets for lenses.

  • Variants include Transmission EM (TEM) and Scanning EM (SEM).

  • Provides ultra-high resolution to view organelle ultrastructure.

The Value of Histology in Diagnosis

• Interprets tissue architecture and cellular details for diagnosis and prognosis.

• Rudolf Virchow's quote: chemical changes precede visible anatomical changes.

• Supports cancer staging, which guides treatment and prognosis.

• Complemented by translational observations (e.g., pattern recognition in pathology via trainable observers).

Biopsy Techniques: Sampling Tissues for Histology

• Types of biopsy

  • Needle biopsy: brain, thyroid, breast, liver, kidney, bone.

  • Endoscopic biopsy: respiratory, gastrointestinal tract.

  • Transvascular biopsy: heart, liver.

  • Direct excision biopsy: skin, mouth, larynx, uterine cervix.

  • Curettage biopsy: endometrial lining.

• Each method chosen based on accessibility, suspected pathology, and patient safety.

Examples of Biopsy Procedures

• Smear samples: cervix, buccal cavity.

• Curettage: endometrial lining.

• Direct incision: skin, mucosal surfaces.

• Needle biopsy: brain, breast, liver, kidney, muscle.

• Endoscopic biopsy: lung, intestine, bladder.

• Transvascular biopsy: heart, liver.

Why Tissue Fixation and Slide Preparation Are Needed

• Fresh tissue is prone to autolysis and putrefaction; fixation preserves structure.

• Fixation chemically preserves tissue by cross-linking proteins and stabilizing cellular components.

• Summary workflow:

  • Biopsy collection → fixation → embedding/processing → dehydration → clearing → embedding (paraffin) → sectioning → staining → coverslipping → viewing/analysis.

Tissue Processing: Fixation, Embedding, Dehydration, Clearing, Sectioning, Staining

• Fixation: Chemical preservation (e.g., glutaraldehyde, formaldehyde, alcohol) of macromolecules and structure.

• Dehydration: Replace water with ethanol (70–100%).

• Clearing: Remove ethanol, make tissue miscible with embedding medium (e.g., xylene).

• Embedding: Impregnate with wax and solidify for sectioning.

• Sectioning: Microtomy to cut thin sections; typical thickness ~$3-5\ \mu\text{m}$.

• Staining: Apply dyes to visualize structures (e.g., H&E, PAS).

• Coverslipping: Mount stained sections for preservation and viewing.

• Biopsy timing and handling considerations: appropriate fixation times important to avoid artefacts.

Artefacts Arising from Tissue Processing

• Can arise at various steps and affect interpretation.

• Examples: Air bubbles, scoring/tearing of sections.

• Recognition is essential to avoid misinterpretation.

Haematoxylin and Eosin (H&E) Staining

• Haematoxylin: Basic dye, stains acidic (basophilic) components blue/purple (e.g., nucleolus, chromatin).

• Eosin: Acidic dye, stains basic (eosinophilic) components pink/red (e.g., cytoplasmic proteins, collagen).

• Provides contrast between nuclei and cytoplasm for general tissue architecture.

H&E Histology in Gastric Mucosa

• Illustrative example: Gastric pit with overlying mucosa; brown/purple nuclei, pink cytoplasm in glandular epithelium.

• Smooth muscle shows purple nuclei; lumen appears pink.

• Basal membrane acts as a structural boundary.

Periodic Acid–Schiff (PAS) Staining

• Special stain highlighting carbohydrates and glycoproteins in magenta.

• Stains:

  • Brush border (microvilli) in intestinal/renal epithelia.

  • Basement membranes (e.g., proximal convoluted tubule).

  • Glomerulus structures.

PAS in Intestinal Villi and Renal Tissue

• Intestinal villi: simple columnar enterocytes with magenta brush border; goblet cells.

• Renal tissue: proximal convoluted tubule basement membrane stained magenta.

Histological Staining Machine and Quality Control (QC)

• Staining machines automate processes.

• QC checks ensure proper sectioning, staining quality, and mounting.

• Essential for reliable diagnostic slides in hospital settings.

Viewing and Digital Analysis of Slides

• High-throughput digital slide scanners digitize slides.

• Image analysis software enables quantitative assessment and remote review.

• Common systems: Aperio AT2, Leica scanners.

Histology in Clinical Anatomy: Pancreas

• Pancreas histology (H&E):

  • Exocrine pancreas with acini (acinar cells).

  • Ducts and adipose tissue.

  • Islets of Langerhans (endocrine pancreas) dispersed within exocrine tissue.

Nuclear Morphology and Chromatin Organization

• Heterochromatin: Tightly packed DNA; intense hematoxylin staining; low transcriptional activity.

• Euchromatin: Less condensed DNA; stains with hematoxylin; higher transcriptional activity; periphery of nucleus and nucleolus often more active.

Nissl Bodies and Neuronal Structure

• Nissl bodies: Basophilic, granular areas representing RER and free ribosomes; sites of protein synthesis.

• Abundant in large neurons (e.g., motor neurons).

• Chromatolysis: Disintegration or dispersal of Nissl bodies after neuronal injury.

Histological Considerations and Conceptual Framing

• Emphasizes understanding tissue architecture, staining patterns, and pathological implications.

• Highlights how planes of section and orientation affect interpretation.

Planes of Section and Tissue Orientation

• Planes: Transverse, longitudinal, oblique.

• Structures (e.g., skin glands) appear differently depending on the plane.

• Understanding these planes aids 3D visualization from 2D sections.

Skin Anatomy and Plane Effects: Cross-Section Examples

• A single gland can produce multiple cross-sectional appearances depending on the plane.

• Learners should anticipate varying appearances.

Sphere and Plane Concepts in Microscopy

• Conceptual tool to understand how 3D structures yield different 2D cross-sections.

• Helps visualize how different planes intersect a spherical structure.

Phase Contrast Microscopy: Principle and Use

• Exploits differences in refractive index to convert phase shifts in light into intensity differences.

• Allows viewing living cells with good contrast without staining.

• Useful for dynamic studies and minimizing sample preparation artifacts.

Brightfield Microscopy vs Phase Contrast

• Brightfield (BF): Basic illumination; often requires staining for contrast.

• Phase-contrast (PC): Enhanced contrast for live cells; often eliminates need for staining.

• PC is useful for observing living cells and quick visual checks.

Confocal Microscopy

• Provides high-resolution images with reduced out-of-focus light.

• Uses fluorescent dyes/labeled antibodies for specific proteins.

• Enables 3D reconstructions from optical sections.

• Applications: detailed localization of molecules within cells and tissues.

Dark-field Microscopy

• Uses scattered light to enhance edges and fine details.

• Useful for detecting pathogens: Treponema pallidum (syphilis), Vibrio cholerae.

• Provides high-contrast images of structures with little or no staining.

Resources and Further Reading

• Junqueira’s Basic Histology, Zeiss Education, Nikon MicroscopyU, Leeds Histology Quiz.

Melanoma Staging and Histology (Dermatology Case Example)

• Histology shows malignant melanocytes; cancer staging informs treatment/prognosis.

• Breslow thickness: Measures depth of invasion from granular layer of epidermis.

  • < 1 mm: 5-year survival ~$95\%-100\%$

  • 1–2 mm: 5-year survival ~$80\%-96\%$

  • 2–4 mm: 5-year survival ~$60\%-75\%$

  • > 4 mm: 5-year survival ~$37\%-50\%$

• Clark's level: Describes invasion depth by skin layers (I–V).

  • I: intra-epidermal.

  • II: invasion into papillary dermis.

  • III: tumor fills papillary dermis.

  • IV: invasion into reticular dermis.

  • V: invasion into subcutaneous fat.

• Deeper invasion correlates with worse prognosis.

Biopsy and Pathology Workflow: From Sampling to Diagnosis

• Key steps recap:

  • Biopsy collection → Fixation → Embedding/Processing → Sectioning → Staining → Coverslipping and QC → Viewing/Analysis.

• Quality of fixation and processing directly impacts diagnostic accuracy; artefact recognition is crucial.

Summary of Stain-Specific Features and Usage

• H&E: General-purpose; nuclei basophilic (blue/purple), cytoplasm/ECM eosinophilic (pink).

• PAS: Highlights carbohydrates/glycoproteins in magenta; useful for brush borders, basement membranes, glycogen-rich cells.

Key Microscopy Concepts and Practical Implications

• Understanding scale and resolution is essential.

• Knowledge of processing helps anticipate artefacts and optimize slide quality.

• Staining patterns provide clues to tissue identity and pathology.

• Modern practice integrates digital slides for diagnosis and education, with QC for reliability.

References and Recommended Study Prompts

• Familiarize with sizes and scales, memorize Breslow thickness, review planes of section, revisit differences among microscopy modalities, relate histology to clinical scenarios.