Microscopy and Cellular Structures

The Worlds of the Micrometer and Nanometer

• Visual scales:

  • Animal cell typical size: $ext{≈}20\,\mu\text{m}$
  • Plant cell vacuole: $20\times 30\,\mu\text{m}$
  • Bacterium typical size: $1\times 2\,\mu\text{m}$
  • Ribosome diameter: $25\text{-}30\,\text{nm}$
  • Membrane thickness: $7\text{-}8\,\text{nm}$
  • Microtubule diameter: $25\,\text{nm}$
  • Microfilament diameter: $7\,\text{nm}$
  • DNA double helix width: $2\,\text{nm}$

• Figure 1-4 (illustrations): shows two scales

  • The world of the micrometer (typical cells and organelles)
  • The world of the nanometer (molecules and subcellular components)

• Examples tying scales together:

  • A bacterium (~$1\times 2\,\mu\text{m}$) sits in the micrometer world, while ribosomes (~$25\text{-}30\,\text{nm}$) sit in the nanometer world.
  • Organelles (nuclei, mitochondria, chloroplasts) occupy the micrometer scale within cells.

Microscopy: The Key Tool in Cytology

• Central technique in cytology for visualizing cells and cellular components at relevant dimensions.

• Two major forms used to study cells:

  • Light microscopy
  • Electron microscopy

• Role of light microscopy:

  • Earliest and still essential tool for identifying membrane-bounded organelles such as nuclei, mitochondria, and chloroplasts.
  • Organelles: "little organs"; prominent in plant and animal cells but not in bacterial cells.
  • Organelles overview introduced here and discussed in more detail in Chapter 4 and later chapters.

• Brightfield microscopy (basic light microscopy):

  • White light passed directly through a specimen.
  • Specimens are either stained or unstained; field is illuminated.
  • Major limitation: many specimens must be chemically fixed, dehydrated, embedded (paraffin or plastic), sectioned into thin slices, and stained to reveal features.
  • Fixed/stained specimens are not alive; slide preparation can distort features and may not reflect living cells.

• Living-cell imaging with light microscopy:

  • To overcome brightfield limitations, several specialized light-microscopy techniques exist for observing living cells directly:
  • Phase-contrast microscopy
  • Differential interference contrast (DIC) microscopy
  • Fluorescence microscopy
  • Confocal microscopy
  • Appendix provides more detail and sample images for these techniques.

Phase-contrast and Differential Interference Contrast (DIC) Microscopy

• Goal: visualize living cells clearly without staining.

• Phase concept:

  • Light waves have crests and troughs; the position of these features is described by the phase of the light.
  • Phase differences arise when light passes through structures of different density than the surrounding medium.

• How these techniques work:

  • Both phase-contrast and DIC convert small phase shifts into differences in light intensity or contrast.
  • This enhancement makes cellular structures with slight density differences more visible in living cells.

Fluorescence Microscopy

• Principle:

  • Detects specific molecules (proteins, DNA sequences, etc.) that have been made fluorescent.
  • Methods to achieve fluorescence:
  • Coupling to a fluorescent dye
  • Using a fluorescent protein
  • Binding to a fluorescently labeled antibody

• Antibodies and fluorescence:

  • An antibody is a protein produced by the immune system that binds to a specific target molecule, known as its antigen.
  • Fluorescent antibodies (immunofluorescence) enable localization of particular molecules within cells.

• Advantages:

  • Allows targeting of specific proteins or nucleic acid sequences within the cellular context.
  • Can be used in fixed samples and, with appropriate conditions, in live cells.

Confocal Microscopy (brief mention)

• Mentioned as one of the living-cell imaging techniques.
• Provides optical sectioning to improve resolution along the z-axis, enabling clearer 3D reconstructions of cellular structures.

Electron Microscopy (context)

• Electron microscopy is noted as the other major form of microscopy beyond light microscopy.
• Provides higher resolution imaging suitable for visualizing smaller features beyond the light microscope’s limit.

Practical and Interpretive Considerations

• Sample preparation implications:

  • Brightfield and staining procedures may distort features of living cells.
  • Fixed and stained specimens do not reflect dynamic processes in live cells.

• Scale and interpretation:

  • Many cellular features exist across a broad size range from nanometers to micrometers.
  • Understanding the scale is essential for selecting the appropriate imaging modality.

• Connections to foundational concepts:

  • The Chapter 4 overview of organelle types provides context for identifying structures in cells.
  • The separation of scales (micrometer vs nanometer) helps frame what can be observed with light vs electron microscopy.

• Real-world relevance:

  • Different microscopy techniques enable researchers to answer questions about structure, localization, and dynamics of cellular components.

• Quick numerical recap (for quick reference):

  • Animal cell size: $ext{≈}20\,\mu\text{m}$
  • Plant cell vacuole: $20\times 30\,\mu\text{m}$
  • Bacterium size: $1\times 2\,\mu\text{m}$
  • Ribosome: $25 ext{-}30\,\text{nm}$
  • Membrane thickness: $7 ext{-}8\,\text{nm}$
  • Microtubule: $25\,\text{nm}$
  • Microfilament: $7\,\text{nm}$
  • DNA double helix: $2\,\text{nm}$