Histology and Cytology Overview

Study Notes

Tissues are often too thick to study directly under a microscope so thin tissue sections must be prepared

Paraffin sections:
- Most common type for Light Microscopy (LM)
- Tissue is embedded in melted paraffin wax
- This allows the tissue to be cut into thin sections
- Disadvantage: High temperatures can dissolve cell lipids and inactive sensitive enzymes
Frozen sections:
- Rapid method for LM
- Tissue is rapidly frozen in liquid nitrogen
- Preserves cell structures and enzymes
- Good for histochemical studies of lipids and enzymes
Plastic sections:
- Used to prepare thin tissue sections for electron microscopic examinations

Most cells and ECM components are colorless
Staining is necessary to differentiate structures under the microscope
General histological methods:
- One dye stains various structures
- E.g. haematoxylin stains the nucleus and cytoplasm
- Hematoxylin: basic dye that stains acidic cell components (nucleic acids)
  - Basophilic: cell components that have an affinity for basic dyes
- Eosin: acidic dye that stains basic cell components (cytoplasm proteins)
  - Acidophilic: cell components that have an affinity for acidic dyes
Specific histological methods:
- One dye stains only one structure or substance
- E.g. Orcein stains only elastic fibers
Vital staining methods:
- Enter cells via diffusion or phagocytosis
- Stain cellular components without harming the cell
- Allow for the study of living cells
Histochemical and cytochemical methods:
- Identify and localize a specific substance (lipids or enzymes)
- Often used with frozen sections
- Chemical methods e.g., P.A.S. reaction for identifying glycogen
- Physical methods e.g. fluorescent substances (vitamin A) can be seen directly with the microscope

Instruments used for inspecting small objects that are invisible to the naked eye
Resolution:
- The microscope's ability to distinguish fine details
- Limit of resolution: smallest diameter of an object that can be visualised
- Human eye: 0.1mm, Light Microscope: 0.2µm, Electron Microscope: 0.2nm
Magnification:
- Increase in the size of an object without optical defects

Use electron beams to study fine tissue structures
Transmission Electron Microscope (TEM):
- Resolving power: 0.2nm
- Magnification: 100,000 times
- High voltage electron microscopy (HVEM): subtype of TEM that can examine thicker tissue sections (5 microns) and study living cells
Scanning Electron Microscope (SEM):
- Produces a three-dimensional image
- Useful for studying surface details (like cilia)