Comprehensive Study Notes on Microscopy and Cell Theory

Historical Development of the Microscope

The development of the microscope is characterized by a series of technological advancements that have significantly increased our ability to visualize the microscopic world.

• 1590 - Hans Janssen and Zacharias Janssen: This father-and-son duo from the Netherlands placed multiple lenses into a tube. They discovered that objects viewed through this tube appeared greatly enlarged, effectively creating an early precursor to the compound microscope.
• 1609 - Galileo Galilei: He invented a compound microscope that utilized a combination of convex and concave lenses.
• 1625 - Giovanni Faber: This was the first time the specific term "microscope" was used. Faber coined the term to refer to the compound microscope developed by Galileo.
• 1665 - Robert Hooke: An English physicist who published Micrographia. He was the first to observe a plant cell (specifically cork) using a single lens microscope. He coined the term "cell" because the structures he saw reminded him of small rooms or "cella."
• 1676 - Antonie van Leeuwenhoek: The first to observe living cells using a single lens microscope of his own design. He famously examined and documented blood cells, yeast, and various insects.
• 1830 - Joseph Lister: He addressed the issue of spherical aberrations by utilizing several weak lenses placed together at specific distances. This allowed for high magnification without the resulting image becoming blurred.
• 1874 - Ernst Abbe: He introduced a mathematical formula correlating resolving power to the wavelength of light. This formula allowed for the calculation of the theoretical maximum resolution of a microscope.
• 1931 - Ernst Ruska and Max Knoll: Developed and built the first transmission electron microscope (TEM). Unlike light microscopes, this device utilizes electrons rather than light, allowing for the visualization of objects as tiny as the diameter of an atom.
• 1932 - Frits Zernike: Invented phase contrast illumination. This allowed for the imaging of transparent samples without the need for staining, which can often kill or alter living specimens.
• 1942 - Ernst Ruska: Invented the first scanning electron microscope (SEM), which transmits a beam of electrons across the surface of a specimen to create a detailed image.
• 1957 - Marvin Minsky: Introduced the principle of confocal imaging, providing a resolution higher than that achievable with conventional light microscopy.
• 1972 - Godfrey Hounsfield and Allan Cormack: Developed the Computerized Axial Tomography (CAT) scanner, which generates cross-sectional views and three-dimensional images of internal structures and organs.
• 1978 - Thomas and Christoph Cremer: Developed the first practical confocal laser scanning microscope, which uses focused laser beams to scan objects.
• 1981 - Gerd Binnig and Heinrich Rohrer: Invented the scanning tunnelling microscope (STEM), capable of visualizing individual atoms within materials.
• 1986 - Nobel Prize Recognition: Ernst Ruska was awarded the Nobel Prize for his contributions to microscopy. Gerd Binnig and Heinrich Rohrer also received a Nobel Prize.
• 1992 - Douglas Prasher: Cloned the green fluorescent protein (GFP), which became a vital tool in fluorescence microscopy.
• 1993-1996 - Stefan Hell: Pioneered the first super-resolution microscopy techniques.
• 2008 - Titan 80-300 Cubed: The first installation of this electron microscope in Canada revolutionized nanotechnology by allowing researchers to explore new frontiers.
• 2010 - UCLA Researchers: Used a cryoelectron microscope to visualize the individual atoms of a virus.
• 2014 - Nobel Prize in Chemistry: Awarded to Eric Betzig, Stefan Hell, and William Moerner for their invention of "super microscopes" capable of seeing matter smaller than $0.2\,\text{micrometers}$.
• 2017 - Jacques Dubochet, Joachim Frank, and Richard Henderson: Developed a technique for generating 3D structures of proteins at an atomic level using electron microscopy.
• 2018 - Titan Krios: A high-end transmission electron microscope developed by Thermo Fisher Scientific was officially inaugurated, advancing the transition from 2D to 3D imaging with Cryo-EM.

The Parts of the Microscope and Their Functions

Most classroom laboratories use light compound microscopes to observe living or preserved specimens, such as bacteria. These microscopes typically magnify specimens up to $1000$ times their original size. Stains are frequently used to make specific images stand out.

Magnification and Optics

• Total Magnification Formula: The total magnification of a specimen is calculated by multiplying the magnification of the ocular lens by the magnification of the objective lens.
  • $\text{Ocular lens} \times \text{Objective lens} = \text{Total magnification}$
• Examples:
  • $10\times \times 10\times = 100\times$ (Specimen appears $100$ times larger).
  • $10\times \times 40\times = 400\times$ (Specimen appears $400$ times larger).
• Resolution: The ability of a microscope to show the fine details of an object. It is defined as the shortest distance between two points on a specimen that can still be distinguished as separate entities by the observer.
• Contrast: Refers to the darkness of the background relative to the specimen. Light specimens are often viewed more clearly against darker backgrounds. For transparent or colorless specimens, a phase contrast microscope is used.

Mechanical Parts

These parts provide support, strength, and adjustability to the instrument.

• Body Tube: A hollow tube through which light travels from the objective lens to the eyepiece.
• Revolving Nosepiece: Holds the objective lenses. It can be rotated to select different magnifications; the lenses must "click" into place.
• Arm: Connects the base to the body tube and serves as the handle for carrying the microscope.
• Stage: The platform where the slide or specimen is placed for examination. It contains an opening in the center to allow light to pass through.
• Stage Clips: Used to hold the slide securely in place on the stage.
• Base: The bottom part that firmly anchors and supports the entire microscope; it is where the illuminators are attached.
• Inclination Joint: Found in some models; it is a joint where the arm attaches to the pillar, allowing the microscope to be tilted for more comfortable viewing.

Illuminating Parts

These parts capture and provide the light necessary to view the specimen.

• Mirror: Reflects light from the environment up to the specimen. It has two sides: a planar (flat) side for artificial light and a concave side for natural light. Modern microscopes often replace this with a built-in light bulb or source.
• Condenser: Concentrates and focuses the light from the source onto the specimen. It is located beneath the stage.
• Iris Diaphragm: Located beneath the condenser, it regulates the specific amount of light that reaches the specimen.

Magnifying Parts

These parts focus and enlarge the image of the specimen.

• Eyepiece (Ocular): The lens the observer looks through. It typically has a magnification of $10\times$, though variations ranging from $5\times$ to $30\times$ exist.
• Objectives: The primary lenses used for magnification. Standard compound microscopes usually have three to five objectives with typical powers of $4\times$, $10\times$, $40\times$, and $100\times$.

Cell Theory: The Unifying Foundation of Cell Biology

In the 1820s, advancements in lens design allowed for the detailed observation of internal cell structures.

• Robert Brown (1820s): A botanist who first observed and named the "nucleus," the spherical structure found within plant cells.
• Theodore Schwann (1839): A zoologist who discovered that animal tissues are composed of cells.
• Matthias Schleiden (1839): A botanist who concluded that all plant tissues are composed of cells.
• Rudolf Virchow (Approx. 1850s): A physician who studied cell growth and development, concluding that all cells arise from preexisting cells.

The Tenets of Cell Theory

The original observations of Schwann, Schleiden, and Virchow form the three classic tenets of cell theory:

1. Cells are the smallest unit of life. All living things are composed of one or more cells.
2. Cells are the basic unit of organization of all organisms.
3. Cells come only from preexisting cells.

Modern cell theory adds two additional points:

4. Cells carry and pass hereditary units (DNA) to offspring during cell division.
5. All cells are relatively similar in chemical composition and metabolic activity.

Cell Diversity: Size, Shape, and Organization

Cell Size

Most cells are microscopic, though some exceptions like unfertilized bird eggs are large enough to see with the naked eye.

• Bacterial Cells: Range from approximately $1$ to $10\,\mu m$ (microns) in diameter.
• Plant and Animal Cells: Usually range from $10$ to $50\,\mu m$ in diameter.

Cell Shape

A cell's shape is determined by its specific function:

• Nerve Cells (Neurons): Long structures with cytoplasmic extensions (axons and dendrites) designed to transmit impulses from the central nervous system throughout the body.
• Skin Cells: Flat cells that provide a protective covering for the body.
• Blood Cells: Can change shape, which assists them in digesting and killing disease-causing germs.

Internal Organization

The structural characteristics of a cell relate to its functional role:

• Species Variation: Plant and animal cells differ significantly because they perform different tasks.
• Specialization within Organisms: Human body cells are specialized for specific jobs.
  • Glandular Cells: Produce secretory materials like mucus and hormones; they contain more ribosomes and Golgi bodies.
  • Muscle Cells: Contain a high number of mitochondria to provide the energy required for muscle contraction.

Questions & Discussion

Quiz 1.1

1. Who is considered the ‘English Father of Microscopy’?
   • a. Robert Hooke (Correct Answer)
   • b. Hans Janssen
   • c. Robert Brown
   • d. Rudolf Virchow
2. Which of the following is not a tenet of the cell theory?
   • a. All living things are made up of cells.
   • b. All living things are composed of atoms. (Correct Answer - this is atomic theory, not cell theory)
   • c. All cells come from preexisting cells.
   • d. Cells are the basic functional unit of life.
3. What part of the microscope focuses the light on the specimen being observed?
   • a. Mirror
   • b. Objective lens
   • c. Condenser (Correct Answer)
   • d. Ocular
4. Who was the Dutch microscope maker who pioneered the study of protozoa?
   • a. Louis Pasteur
   • b. Robert Hooke
   • c. Galileo Galilei
   • d. Anton van Leeuwenhoek (Correct Answer)
5. Which of the following is measured in cubic centimeters?
   • a. Area
   • b. Volume (Correct Answer)
   • c. Weight
   • d. Height

Application and Analysis

1. Why are the cells of giraffes not larger than those of a mouse?
2. Leeuwenhoek’s microscope revealed microscopic organisms he called ‘animalcules’, yet he never concluded that the cells are the smallest unit of living organism. What could be the reason for this?
3. It is said that the cell theory is a cornerstone in biology. Why do you think this is so?