Microscopy techniques

Light Microscopy - The Basic Toolkit

• Principle: Uses visible light passed through a specimen and glass lenses. Total magnification = (Objective lens) x (Eyepiece lens).
• Resolution: The ability to distinguish between two close points. Limited by the wavelength of light. Max resolution ≈ 0.2 µm.
  • Formula: $R = 0.61λ / NA$ (where λ = wavelength, NA = numerical aperture).
• Common Stains (H&E):
  • Hematoxylin (H): Basophilic. Stains acidic, negatively charged structures (e.g., DNA, RNA, ribosomes) a purplish-blue.
  • Eosin (E): Acidophilic/Eosinophilic. Stains basic, positively charged structures (e.g., cytoplasm, collagen) a reddish-pink.
  • 📌 Mnemonic: BASophilic = Blue; Acidophilic = piNK.

⭐ The absolute limit of resolution for light microscopy is ~200 nm. Therefore, structures smaller than this (e.g., individual ribosomes, viruses, macromolecules) cannot be visualized.

Electron Microscopy - Ultimate Zoom Lens

• Principle: Utilizes a focused beam of electrons instead of light, achieving significantly higher resolution (down to 0.1 nm) to visualize cellular ultrastructure.
• Types:
  • Transmission EM (TEM): Provides high-magnification, 2D images of a specimen's internal composition. Electrons are transmitted through an ultra-thin slice. Essential for viewing organelles, viruses, and molecular complexes.
    • 📌 Mnemonic: TEM = Transmits Through.
  • Scanning EM (SEM): Generates lower-magnification, 3D images of a specimen's surface topography. An electron beam scans the surface, causing secondary electron emission.
    • 📌 Mnemonic: SEM = Surface Scanning.

⭐ EM is the gold standard for diagnosing diseases involving ultrastructural changes, such as Minimal Change Disease, by visualizing the effacement of podocyte foot processes in the glomerulus.

Special Techniques - Seeing the Unseen

• Immunofluorescence (IF): Visualizes antigens using fluorescently-tagged antibodies.

  • Direct IF: Labeled primary antibody.
  • Indirect IF: Labeled secondary antibody; offers ↑ signal amplification.

• Immunohistochemistry (IHC): Uses an enzyme-linked antibody to create a visible colored product (e.g., brown). Routinely used to classify tumors.

• Electron Microscopy (EM): High-resolution view of cellular ultrastructure.

  • Transmission (TEM): 2D view of internal structures (organelles).
  • Scanning (SEM): 3D surface view.

• Flow Cytometry: Analyzes properties of single cells in suspension using laser optics. Identifies cell surface markers (e.g., CD markers).

• In Situ Hybridization (ISH): Detects specific DNA/RNA sequences in tissues using a labeled probe. Used for viral detection (CMV, HPV) or gene localization.

⭐ Flow cytometry is essential for diagnosing and classifying leukemias/lymphomas by detecting specific lymphocyte CD markers.

Artifacts - When Glitches Happen

• Shrinkage: Fixation/dehydration causes artificial spaces between cells and stroma.
• Folds/Wrinkles: Tissue folding during mounting creates dense, dark lines obscuring details.
• Knife Marks: Parallel lines/tears from a nicked microtome blade.
• Air Bubbles: Round, clear spaces under the coverslip that refract light.
• Ice Crystal Artifact: "Swiss cheese" holes in frozen sections from ice formation.

⭐ Formalin pigment, a brown/black granular deposit, can mimic hemosiderin. It's an acid hematin artifact, preventable with buffered formalin.

High‑Yield Points - ⚡ Biggest Takeaways

• Hematoxylin (blue) stains basophilic structures (e.g., DNA); Eosin (pink) stains acidophilic proteins.
• Electron microscopy offers supreme resolution: TEM for internal ultrastructure, SEM for 3D surface views.
• Immunofluorescence uses fluorophore-tagged antibodies to localize specific proteins.
• Flow cytometry counts and sorts cells, often using CD markers for identification.
• In Situ Hybridization (ISH) detects specific DNA or RNA sequences directly in tissue.
• Darkfield microscopy is vital for visualizing unstained live organisms like spirochetes.