Fluorescence Microscopy and Immunofluorescence Imaging

Introduction to Fluorescence Microscopy

  • Transmitted-light microscopy:

    • Visible light passes through a specimen, revealing structures by absorbing and bending light.
    • Histochemistry: Enhances visualization through staining with colored dyes.

  • Fluorescence microscopy:

    • Structures are visualized based on light emission from the specimen.
    • Fluorescence phenomenon:
    • Fluorophore captures electromagnetic radiation (excitation wavelength) and re-emits at lower energy (emission wavelength).
    • Example:
      • Fluorescein isothiocyanate (FITC) absorbs light at 488extnm488 ext{ nm} and emits at 525extnm525 ext{ nm}.

Key Components

  • Epifluorescence Microscopy:

    • Specimen is illuminated with excitation light from the viewing side.
    • Emitted light passes through the optical system to an eye or camera.
    • Dichroic mirror: Reflects excitation light and permits emitted wavelengths to pass.

  • Emission Filters:

    • Enhance sensitivity by allowing only desired emission wavelengths to pass.
    • Multiple filters enable selection of various excitation and emission wavelengths.

Imaging Multiple Fluorophores

  • Traditional fluorescence microscopes can only image one fluorophore at a time:
    • Multiple images must be recorded and digitally combined to create a composite image.
  • Digital cameras often record in grayscale due to weak emissions, with false colors assigned later.
  • Single-color LEDs: Current advancements allow imaging of multiple fluorophores simultaneously, but may cause signal bleeding.

Channel Imaging and Presentation

  • Retaining separate channel images improves detail visibility in analysis.
  • Scientists frequently display both grayscale images and composite color images in publications.

Limitations of Epifluorescence Microscopy

  • Difficulty in determining co-localization of fluorophores:
    • Overlapping signals could mislead, suggesting proximity when fluorophores are actually distant in three-dimensional space.

Confocal Microscopy

  • Addresses limitations by using:
    • Pinhole aperture: Reduces background fluorescence, requiring longer exposure.
    • Laser excitation: Targets specific areas, creating virtual sections.
    • Z stack creation: Sequential sections combined for a 3D image visualization.

Immunofluorescence Imaging Procedure

  • Retrieve and prepare slides protected from bright light to prevent photobleaching.
  • Focus using the DAPI channel first, as it photobleaches slower.
  • Capture images of specific cellular structures using appropriate objectives (40x or 100x).
    • Look for cilia, macronucleus, and micronucleus.
  • Save images in a defined format (e.g., Aneesha_Naomi_DAPI_40x.jpg).

Example of Composite Imaging

  • Final merged image shows immunostaining:
    • Tubulin detected with FITC-conjugated antibody (green).
    • Nuclei stained with DAPI (blue).
    • Composite created using ImageJ JS.