Introduction to Cytology and Microscopes

Introduction to Cytology and Genetics

• Cytology and genetics are introduced.
• Key cell components are shown in a diagram.
  • Cell membrane
  • Mitochondrion
  • Cytoplasm
  • Nucleus
  • DNA
  • Endoplasmic reticulum
  • Lysosome
  • Ribosome
  • Golgi apparatus

Microscopes

• Microscopes are optical devices used to view objects too small to be seen with the naked eye (less than 70µm).
• The main goal of the chapter is to provide basic information about microscope types, designs, principles of operation, and usage in biomedical disciplines.

Types of Microscopes

• Light microscopes and electron microscopes are distinguished by the type of radiation used.

Light Microscopes

• Light microscopes use white or ultraviolet light (sunlight, bulbs, vapor lamps) as their light source.
• Optical parts are made of cut glass.
• The resolution is 0.2 µm with a maximum theoretical magnification of 2,000 times.
• Practically, magnification is usually up to 1,000 times.
• In routine conditions, slides (native or fixed) are observed using transmitted light.
• Slides can also be illuminated from above, mainly for fluorescence and inverted microscopy.

Light Microscope Components

• Eyepieces
• Observation tube
• Objective lens
• Stage
• Condenser lens
• Light source
• Base
• Neck
• Coaxial stage controls
• Course focus
• Fine focus
• Switch
• Rheostat

Electron Microscopes

• Electron microscopes use a stream of electrons emitted by a cathode as radiation.
• Optical parts are specialized electromagnetic lenses.
• Resolution is 0.2 nm, with a maximal useful magnification up to 1,000,000 times.
• Slides require special preparation (fixation, staining, contrasting, etc).
• Distinguished by transmission electron microscopes (TEM) and scanning electron microscopes (SEM).
  • TEM: electron beam passes through the sample.
  • SEM: electron beam scans the surface of the sample.

Types of Electron Microscopes

• Transmission Electron Microscope (TEM): used for high-resolution images of internal cell structures.
• Scanning Electron Microscope (SEM): produces a 3D image of the specimen's surface, allowing visualization of surface details, shapes, and sizes.

Types of Light Microscopes: Stereomicroscope

• Also called dissecting microscopes.
• Use two light sources focused on the same point from slightly different angles, allowing 3D viewing of specimens.
• Relatively low magnification (typically less than 100x).
• Can have single or multiple discrete magnification or zoom options.
• Has a longer working distance than typical microscopes.

Types of Light Microscopes: Inverted Microscope

• A special type of light microscope with a reversed optical path.
• Optical parts and the light source are positioned below the specimen (and slide).
• Primarily used for viewing and monitoring cell cultures growing on the bottom of culture vessels (e.g., Petri dishes).

Fluorescence Microscope

• A type of light microscope that uses ultraviolet (UV) radiation for excitation and specific wavelengths of visible light for emission.
• Used for specimens treated with fluorescent dyes (fluorochromes).
• Useful for molecular cytogenetics (e.g., FISH).
• Only a few objects naturally fluoresce.

Polarized Microscope

• Uses polarized light generated by special Nicol prisms.
• Used for observing structures like chitin, cellular fibers, and crystalline inclusions.

Recommended Procedure for Microscopic Observation

• Positioning a specimen in a microscope can be done using quadrant, concentric circles, or clock-face methods.

Types of Slide Preparations

• Impression preparations: a slide lightly pushed on a specimen's surface.
• Smears: a small drop of cell suspension spread on a slide.
• Covered slides: cell suspension or processed histology tissue on a slide with a cover slip.

Native Slides

• Used to observe physiological manifestations of cells (movement, division, ingestion).

Permanent Slides

• Detailed observation of cell morphology following cell or tissue fixation and staining.

Fixation

• Terminates ongoing living processes and autolysis in cells.
• Methods include chemical fixation (e.g., formalin, methanol, ethanol) and physical fixation (heating, drying).

Staining

• Follows fixation, using stains based on affinity interactions between components and staining solutions.

The Cell

• The cell is the basic morphological and functional unit of all organisms (unicellular and multicellular).
• Autonomous and dynamic systems characterized by life processes including metabolism, growth, irritability, and reproduction.

Cell Theory

• Schleiden and Schwann (1838): Plant and animal cells are fundamental components of all living organisms.
• Virchow (1855): Revised cell theory summarizing into three points: 1) The cell is the basic unit of all organisms; 2) Every cell is composed of a nucleus and cytoplasm; 3) Every cell arises from a pre-existing cell ("omnis cellula e cellula").

Cell Types: Prokaryotic and Eukaryotic Cells

• Prokaryotic cells are small, simple unicellular organisms (bacteria and archaea), lacking a membrane-bound nucleus and other organelles.
• Eukaryotic cells are larger and complex, typically multicellular organisms, with a membrane-bound nucleus and various organelles.
• Similarities between both types of cells are a cell (plasma) membrane, cytoplasm, ribosomes, and DNA.

Eukaryotic Cell (General description)

• Eukaryotic cells form unicellular or multicellular organisms;
• Distinguished by their nutritional mode/type. (autotrophic or heterotrophic).

Shape and Size of Cells

• Cell shape and size are genetically determined and related to location and function.
• Cell shapes vary widely (spherical, biconcave disc, squamous, cuboidal, columnar, polygonal, spindle-shaped, multi-polar, pear-shaped).
  • Projections like cilia, microvilli, and flagella can also be present.

Cell Size

• Cells fall into three categories:
  • Small: typically 10µm or less (erythrocytes, lymphocytes).
  • Medium: 10-30µm (plasma cells, chondrocytes).
  • Large: over 30µm (ova, megakaryocytes, motor neurons).

Molecular Structure of Cell Membranes

• Cell membranes (biomembranes) are crucial components of all cells.
• Their discovery significantly relied on advancements in electron microscopy, particularly in the visualization of their trilaminar structure.

Function of Cell Membranes

• Cell membranes separate intracellular from extracellular spaces.
• They are a selectively permeable boundary ensuring dynamic equilibrium between the cell and its environment.
• Membranes contain enzymes, receptors, transport proteins, signalling systems and antigens.

Main Components of Cell Membranes

• Phospholipids are the primary components.
• Phospholipids form a bilayer in aqueous environments, with their hydrophilic (polar) heads facing outward and hydrophobic (non-polar) tails facing inward.
• The lateral movement of phospholipids is possible.

Cell Membrane Proteins

• Integral proteins: penetrate or span the phospholipid bilayer, crucial to transport and other functions.
• Peripheral proteins: reside on the exterior surface of the bilayer, involved in various functions including signal reception.

Cell Membrane Junctions (shown in diagram)

• Zonula occludens (tight junctions)
• Zonula adherens
• Macula adherens (desmosomes)
• Gap junctions

Explore the fundamental concepts of cytology and the various types of microscopes. This quiz covers key cell components and the principles of optical devices used for viewing microscopic objects. Test your knowledge on the operational principles of light and electron microscopes.