Microscopy History, Sample Prep, and Cell Types

Posted on Mar 31, 2025 in Biology

History of Optical Microscopy

Key Milestones

  • 1611: Johannes Kepler suggests methods for constructing a compound microscope.
  • 1655: Robert Hooke publishes Micrographia, introducing the concept of the biological ‘cell’.
  • Mid-17th Century: Antonie van Leeuwenhoek builds over 200 simple microscopes (single high-magnification lenses, up to 270x).
  • Early 19th Century: Giovanni Battista Amici makes significant improvements by correcting spherical and chromatic aberrations in lenses.
  • Late 19th Century: Ernst Abbe develops the apochromatic objective lens, further enhancing image quality.
  • Late 19th Century: Numerous discoveries are made, including descriptions of various tissue types, the cell nucleus, mitochondria in muscle cells, chromosome behavior during mitosis in animal cells, identification of bacteria causing tuberculosis and cholera, and the description of the Golgi apparatus.
  • 1838-1839: Matthias Schleiden and Theodor Schwann formally propose the Cell Theory.

Early Optical Microscopes

In 1655, Robert Hooke observed a thin slice of cork using a microscope he constructed. He coined the term ‘cell’ to describe the small, enclosed compartments he saw. Concurrently, Antonie van Leeuwenhoek crafted numerous simple microscopes, essentially powerful magnifying glasses. These consisted of a copper plate holding a tiny glass bead lens, through which specimens, held steady by a needle-like mount, could be observed at magnifications up to 270 times.

Nineteenth-Century Advancements

In the early nineteenth century, lens quality significantly improved. Giovanni Battista Amici tackled spherical and chromatic aberrations. Later, Ernst Abbe designed the first apochromatic objective lens, minimizing color distortions. The late nineteenth century saw a surge in microscopic discoveries by many scientists, detailing cellular components and processes.

Sample Preparation Techniques

Light Microscopy Sample Preparation

  1. Samples often need to be stained with specific dyes to make structures visible.
  2. Before staining, fixation is frequently required. This process preserves the sample, prevents deterioration, and aids dye penetration.
  3. To avoid distortions caused by fixation and embedding, samples can sometimes be rapidly frozen. Sectioning then requires a specialized microtome called a cryostat.
  4. Mounting is the final step. It can be temporary or permanent. Depending on the specimen and observation needs, mounting media can be water-soluble or non-soluble.

Electron Microscopy Sample Preparation

Preparation for electron microscopy is more complex, and living material cannot be observed.

  1. Fixation: The sample is treated with agents like glutaraldehyde or osmium tetroxide to stabilize proteins and lipids, preventing structural changes.
  2. Sectioning: The sample is infiltrated with resin and cut into ultrathin sections, typically 50-100 nanometers thick.
  3. Mounting: Serial sections are placed onto a special copper grid.
  4. Staining/Coating: The sample is coated with heavy metals (e.g., lead or uranium) to increase electron scattering and enhance contrast.
  5. Viewing: Electrons pass through the sample. Viewing is indirect, occurring via a fluorescent screen or digital imaging system.

Types of Microscopes

Visible Light Microscopes

  • Traditional Optical Microscope (Bright-field)
  • Polarization Microscope
  • Phase Contrast Microscope
  • Differential Interference Contrast (DIC) Microscope
  • Fluorescence Microscope
  • Confocal Microscope

Electron Microscopes

  • Transmission Electron Microscope (TEM)
  • Scanning Electron Microscope (SEM)

Other Wave-Based Microscopes

  • Infrared Microscope
  • Ultraviolet Microscope
  • Röntgen (X-ray) Microscope
  • Scanning Tunneling Microscope (STM)
  • Atomic Force Microscope (AFM)

The Cell Theory

Modern cell theory states that the cell is the fundamental structural, functional, and genetic unit of all living organisms.

Core Principles

  • Structural Unit: All living things are composed of one or more cells; the cell is the smallest unit of life.
  • Functional Unit: Cells carry out the essential processes of life, including metabolism, response to stimuli, and interaction with their environment.
  • Genetic Unit: All cells arise from pre-existing cells through division and contain hereditary information (DNA) passed from cell to cell. From an evolutionary perspective, all cells trace back to the earliest cells on Earth, appearing approximately 3.5 billion years ago.

Fundamental Cell Types

Prokaryotic Cells

  • Genetic material (DNA) is located in the cytoplasm in a region called the nucleoid; it is not enclosed by a nuclear membrane.
  • Lack membrane-bound organelles; the cytoplasm is not compartmentalized into distinct functional spaces.
  • Organisms in the domains Bacteria and Archaea (formerly Kingdom Monera) possess this cell type.

Eukaryotic Cells

  • Possess a true nucleus, where genetic material is enclosed within a double membrane (the nuclear envelope).
  • Contain various membrane-bound organelles within the cytoplasm (e.g., mitochondria, endoplasmic reticulum, Golgi apparatus), each performing specific functions.
  • Organisms in the Kingdoms Protista, Fungi, Animalia, and Plantae are composed of eukaryotic cells, although specific features vary among these groups.